[PATCH RFC v2 0/3] improve aio-polling efficiency

[PATCH RFC v2 0/3] improve aio-polling efficiency

[Jaehoon Kim]

Dear all,

This is v2 of the RFC patch series to refine aio_poll adaptive
polling logic for better CPU efficiency.

v1: 
https://lore.kernel.org/qemu-devel/20260113174824.464720-1-jhkim@linux.ibm.com/

Changes in v2:
- Patch 2/3: Changed default POLL_WEIGHT_SHIFT from 2 to 3
  based on extensive testing. Updated commit message with
  detailed performance comparison showing weight=3 provides
  better balance between throughput and CPU savings

- Patch 3/3: Added proper initialization of poll-weight,
  poll-grow, and poll-shrink defaults in iothread.c

This patch series refines the aio_poll adaptive polling logic to
reduce unnecessary busy-waiting and improve CPU efficiency.

The first patch prevents redundant polling time calculation when
polling is disabled. The second patch enhances the adaptive polling
mechanism by dynamically adjusting the iothread's polling duration
based on event intervals measured by individual AioHandlers. The
third patch introduces a new 'poll-weight' parameter for runtime
control over how much the current interval influences the next
polling duration.

We evaluated the patches on s390x hosts with different configurations:

Initial testing (Fedora 42):
Using a single guest with 16 virtio block devices backed by FCP
multipath devices, I/O scheduler set to 'none'. Across four FIO
workload patterns (sequential R/W, random R/W), averaged over
numjobs 1, 4, 8, and 16:

 - Throughput: -3% to -8% (one iothread), -2% to -5% (two iothreads)
 - CPU usage: -10% to -25% (one iothread), -7% to -12% (two iothreads)

Additional validation (RHEL 10.1 GA + QEMU 10.0.0):
Comparing baseline vs poll-weight=2/3 with FCP and FICON storage,
using 1 and 8 iothreads, averaged over numjobs 1, 4, and 8:

Summary of results (% change vs baseline):

 - Throughput avg: -2.2% (weight=3), -2.4% (weight=2)
 - CPU consumption avg: -9.4% (weight=3), -10.9% (weight=2)

Weight=3 was selected as default for providing slightly better
throughput (-2.2% vs -2.4%) while maintaining substantial CPU
savings (-9.4%).

Best regards,
Jaehoon Kim

Jaehoon Kim (3):
  aio-poll: avoid unnecessary polling time computation
  aio-poll: refine iothread polling using weighted handler intervals
  qapi/iothread: introduce poll-weight parameter for aio-poll

 include/qemu/aio.h                |   8 +-
 include/system/iothread.h         |   1 +
 iothread.c                        |  34 ++++++-
 monitor/hmp-cmds.c                |   1 +
 qapi/misc.json                    |   7 ++
 qapi/qom.json                     |   8 +-
 qemu-options.hx                   |   7 +-
 tests/unit/test-nested-aio-poll.c |   2 +-
 util/aio-posix.c                  | 141 ++++++++++++++++++++----------
 util/aio-win32.c                  |   3 +-
 util/async.c                      |   2 +
 11 files changed, 161 insertions(+), 53 deletions(-)

-- 
2.50.1

[PATCH RFC v2 1/3] aio-poll: avoid unnecessary polling time computation

[Jaehoon Kim]

Nodes are no longer added to poll_aio_handlers when adaptive polling is
disabled, preventing unnecessary try_poll_mode() calls. Additionally,
aio_poll() skips try_poll_mode() when timeout is 0.

This avoids iterating over all nodes to compute max_ns unnecessarily
when polling is disabled or timeout is 0.

Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
---
 util/aio-posix.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/util/aio-posix.c b/util/aio-posix.c
index 488d964611..b02beb0505 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -307,9 +307,8 @@ static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node)
      * fdmon_supports_polling(), but only until the fd fires for the first
      * time.
      */
-    if (!QLIST_IS_INSERTED(node, node_deleted) &&
-        !QLIST_IS_INSERTED(node, node_poll) &&
-        node->io_poll) {
+    if (ctx->poll_max_ns && !QLIST_IS_INSERTED(node, node_deleted) &&
+        !QLIST_IS_INSERTED(node, node_poll) && node->io_poll) {
         trace_poll_add(ctx, node, node->pfd.fd, revents);
         if (ctx->poll_started && node->io_poll_begin) {
             node->io_poll_begin(node->opaque);
@@ -631,7 +630,7 @@ static void adjust_polling_time(AioContext *ctx, AioPolledEvent *poll,
 bool aio_poll(AioContext *ctx, bool blocking)
 {
     AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
-    bool progress;
+    bool progress = false;
     bool use_notify_me;
     int64_t timeout;
     int64_t start = 0;
@@ -656,7 +655,9 @@ bool aio_poll(AioContext *ctx, bool blocking)
     }
 
     timeout = blocking ? aio_compute_timeout(ctx) : 0;
-    progress = try_poll_mode(ctx, &ready_list, &timeout);
+    if ((ctx->poll_max_ns != 0) && (timeout != 0)) {
+        progress = try_poll_mode(ctx, &ready_list, &timeout);
+    }
     assert(!(timeout && progress));
 
     /*
-- 
2.50.1

[PATCH RFC v2 2/3] aio-poll: refine iothread polling using weighted handler intervals

[Jaehoon Kim]

Refine adaptive polling in aio_poll by updating iothread polling
duration based on weighted AioHandler event intervals.

Each AioHandler's poll.ns is updated using a weighted factor when an
event occurs. Idle handlers accumulate block_ns until poll_max_ns and
then reset to 0, preventing sporadically active handlers from
unnecessarily prolonging iothread polling.

The iothread polling duration is set based on the largest poll.ns among
active handlers. The shrink divider defaults to 2, matching the grow
rate, to reduce frequent poll_ns resets for slow devices.

The default weight factor (POLL_WEIGHT_SHIFT=3, meaning the current
interval contributes 12.5% to the weighted average) was selected based
on extensive testing comparing QEMU 10.0.0 baseline vs poll-weight=2
and poll-weight=3 across various workloads.

The table below shows a comparison between:
-Host: RHEL 10.1 GA + qemu-10.0.0-14.el10_1, Guest: RHEL 9.6GA vs.
-Host: RHEL 10.1 GA + qemu-10.0.0-14.el10_1 (w=2/w=3), Guest: RHEL 9.6GA
for FIO FCP and FICON with 1 iothread and 8 iothreads.
The values shown are the averages for numjobs 1, 4, and 8.

Summary of results (% change vs baseline):

                    | poll-weight=2      | poll-weight=3
--------------------|--------------------|-----------------
Throughput avg      | -2.4% (all tests)  | -2.2% (all tests)
CPU consumption avg | -10.9% (all tests) | -9.4% (all tests)

Both weight=2 and weight=3 show significant CPU consumption reduction
(~10%) compared to baseline, which addresses the CPU utilization
regression observed in QEMU 10.0.0. The throughput impact is minimal
for both (~2%).

Weight=3 is selected as the default because it provides slightly better
throughput (-2.2% vs -2.4%) while still achieving substantial CPU
savings (-9.4%). The difference between weight=2 and weight=3 is small,
but weight=3 offers a better balance for general-purpose workloads.

Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
---
 include/qemu/aio.h |   4 +-
 util/aio-posix.c   | 135 +++++++++++++++++++++++++++++++--------------
 util/async.c       |   1 +
 3 files changed, 99 insertions(+), 41 deletions(-)

diff --git a/include/qemu/aio.h b/include/qemu/aio.h
index 8cca2360d1..6c77a190e9 100644
--- a/include/qemu/aio.h
+++ b/include/qemu/aio.h
@@ -195,7 +195,8 @@ struct BHListSlice {
 typedef QSLIST_HEAD(, AioHandler) AioHandlerSList;
 
 typedef struct AioPolledEvent {
-    int64_t ns;        /* current polling time in nanoseconds */
+    bool has_event; /* Flag to indicate if an event has occurred */
+    int64_t ns;     /* estimated block time in nanoseconds */
 } AioPolledEvent;
 
 struct AioContext {
@@ -306,6 +307,7 @@ struct AioContext {
     int poll_disable_cnt;
 
     /* Polling mode parameters */
+    int64_t poll_ns;        /* current polling time in nanoseconds */
     int64_t poll_max_ns;    /* maximum polling time in nanoseconds */
     int64_t poll_grow;      /* polling time growth factor */
     int64_t poll_shrink;    /* polling time shrink factor */
diff --git a/util/aio-posix.c b/util/aio-posix.c
index b02beb0505..2b3522f2f9 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -29,9 +29,11 @@
 
 /* Stop userspace polling on a handler if it isn't active for some time */
 #define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
+#define POLL_WEIGHT_SHIFT   (3)
 
-static void adjust_polling_time(AioContext *ctx, AioPolledEvent *poll,
-                                int64_t block_ns);
+static void adjust_block_ns(AioContext *ctx, int64_t block_ns);
+static void grow_polling_time(AioContext *ctx, int64_t block_ns);
+static void shrink_polling_time(AioContext *ctx, int64_t block_ns);
 
 bool aio_poll_disabled(AioContext *ctx)
 {
@@ -373,7 +375,7 @@ static bool aio_dispatch_ready_handlers(AioContext *ctx,
          * add the handler to ctx->poll_aio_handlers.
          */
         if (ctx->poll_max_ns && QLIST_IS_INSERTED(node, node_poll)) {
-            adjust_polling_time(ctx, &node->poll, block_ns);
+            node->poll.has_event = true;
         }
     }
 
@@ -560,18 +562,13 @@ static bool run_poll_handlers(AioContext *ctx, AioHandlerList *ready_list,
 static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
                           int64_t *timeout)
 {
-    AioHandler *node;
     int64_t max_ns;
 
     if (QLIST_EMPTY_RCU(&ctx->poll_aio_handlers)) {
         return false;
     }
 
-    max_ns = 0;
-    QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
-        max_ns = MAX(max_ns, node->poll.ns);
-    }
-    max_ns = qemu_soonest_timeout(*timeout, max_ns);
+    max_ns = qemu_soonest_timeout(*timeout, ctx->poll_ns);
 
     if (max_ns && !ctx->fdmon_ops->need_wait(ctx)) {
         /*
@@ -587,46 +584,98 @@ static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
     return false;
 }
 
-static void adjust_polling_time(AioContext *ctx, AioPolledEvent *poll,
-                                int64_t block_ns)
+static void shrink_polling_time(AioContext *ctx, int64_t block_ns)
 {
-    if (block_ns <= poll->ns) {
-        /* This is the sweet spot, no adjustment needed */
-    } else if (block_ns > ctx->poll_max_ns) {
-        /* We'd have to poll for too long, poll less */
-        int64_t old = poll->ns;
-
-        if (ctx->poll_shrink) {
-            poll->ns /= ctx->poll_shrink;
-        } else {
-            poll->ns = 0;
-        }
+    /*
+     * Reduce polling time if the block_ns is zero or
+     * less than the current poll_ns.
+     */
+    int64_t old = ctx->poll_ns;
+    int64_t shrink = ctx->poll_shrink;
 
-        trace_poll_shrink(ctx, old, poll->ns);
-    } else if (poll->ns < ctx->poll_max_ns &&
-               block_ns < ctx->poll_max_ns) {
-        /* There is room to grow, poll longer */
-        int64_t old = poll->ns;
-        int64_t grow = ctx->poll_grow;
+    if (shrink == 0) {
+        shrink = 2;
+    }
 
-        if (grow == 0) {
-            grow = 2;
-        }
+    if (block_ns < (ctx->poll_ns / shrink)) {
+        ctx->poll_ns /= shrink;
+    }
 
-        if (poll->ns) {
-            poll->ns *= grow;
-        } else {
-            poll->ns = 4000; /* start polling at 4 microseconds */
-        }
+    trace_poll_shrink(ctx, old, ctx->poll_ns);
+}
 
-        if (poll->ns > ctx->poll_max_ns) {
-            poll->ns = ctx->poll_max_ns;
-        }
+static void grow_polling_time(AioContext *ctx, int64_t block_ns)
+{
+    /* There is room to grow, poll longer */
+    int64_t old = ctx->poll_ns;
+    int64_t grow = ctx->poll_grow;
 
-        trace_poll_grow(ctx, old, poll->ns);
+    if (grow == 0) {
+        grow = 2;
     }
+
+    if (block_ns > ctx->poll_ns * grow) {
+        ctx->poll_ns = block_ns;
+    } else {
+        ctx->poll_ns *= grow;
+    }
+
+    if (ctx->poll_ns > ctx->poll_max_ns) {
+        ctx->poll_ns = ctx->poll_max_ns;
+    }
+
+    trace_poll_grow(ctx, old, ctx->poll_ns);
 }
 
+static void adjust_block_ns(AioContext *ctx, int64_t block_ns)
+{
+    AioHandler *node;
+    int64_t adj_block_ns = -1;
+
+    QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
+        if (node->poll.has_event) {
+            /*
+             * Update poll.ns for the node with an event.
+             * Uses a weighted average of the current block_ns and the previous
+             * poll.ns to smooth out polling time adjustments.
+             */
+            node->poll.ns = node->poll.ns
+                ? (node->poll.ns - (node->poll.ns >> POLL_WEIGHT_SHIFT))
+                + (block_ns >> POLL_WEIGHT_SHIFT) : block_ns;
+
+            if (node->poll.ns > ctx->poll_max_ns) {
+                node->poll.ns = 0;
+            }
+            /*
+             * To avoid excessive polling time increase, update adj_block_ns
+             * for nodes with the event flag set to true
+             */
+            adj_block_ns = MAX(adj_block_ns, node->poll.ns);
+            node->poll.has_event = false;
+         } else {
+            /*
+             * No event now, but was active before.
+             * If it waits longer than poll_max_ns, poll.ns will stay 0
+             * until the next event arrives.
+             */
+            if (node->poll.ns != 0) {
+                node->poll.ns += block_ns;
+                if (node->poll.ns > ctx->poll_max_ns) {
+                    node->poll.ns = 0;
+                }
+            }
+        }
+    }
+
+    if (adj_block_ns >= 0) {
+        if (adj_block_ns > ctx->poll_ns) {
+            grow_polling_time(ctx, adj_block_ns);
+        } else {
+            shrink_polling_time(ctx, adj_block_ns);
+         }
+     }
+ }
+
 bool aio_poll(AioContext *ctx, bool blocking)
 {
     AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
@@ -723,6 +772,10 @@ bool aio_poll(AioContext *ctx, bool blocking)
 
     aio_free_deleted_handlers(ctx);
 
+    if (ctx->poll_max_ns) {
+        adjust_block_ns(ctx, block_ns);
+    }
+
     qemu_lockcnt_dec(&ctx->list_lock);
 
     progress |= timerlistgroup_run_timers(&ctx->tlg);
@@ -784,6 +837,7 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
 
     qemu_lockcnt_inc(&ctx->list_lock);
     QLIST_FOREACH(node, &ctx->aio_handlers, node) {
+        node->poll.has_event = false;
         node->poll.ns = 0;
     }
     qemu_lockcnt_dec(&ctx->list_lock);
@@ -794,6 +848,7 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
     ctx->poll_max_ns = max_ns;
     ctx->poll_grow = grow;
     ctx->poll_shrink = shrink;
+    ctx->poll_ns = 0;
 
     aio_notify(ctx);
 }
diff --git a/util/async.c b/util/async.c
index 80d6b01a8a..9d3627566f 100644
--- a/util/async.c
+++ b/util/async.c
@@ -606,6 +606,7 @@ AioContext *aio_context_new(Error **errp)
     timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
 
     ctx->poll_max_ns = 0;
+    ctx->poll_ns = 0;
     ctx->poll_grow = 0;
     ctx->poll_shrink = 0;
 
-- 
2.50.1

[PATCH RFC v2 3/3] qapi/iothread: introduce poll-weight parameter for aio-poll

[Jaehoon Kim]

Introduce a configurable poll-weight parameter for adaptive polling
in IOThread. This parameter replaces the hardcoded POLL_WEIGHT_SHIFT
constant, allowing runtime control over how much the most recent
event interval affects the next polling duration calculation.

The poll-weight parameter uses a shift value where larger values
decrease the weight of the current interval, enabling more gradual
adjustments. When set to 0, a default value of 3 is used (meaning
the current interval contributes approximately 1/8 to the weighted
average).

This patch also removes the hardcoded default values for poll-grow
and poll-shrink parameters from the grow_polling_time() and
shrink_polling_time() functions, as these defaults are now properly
initialized in iothread.c during IOThread creation.

Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
---
 include/qemu/aio.h                |  4 +++-
 include/system/iothread.h         |  1 +
 iothread.c                        | 34 ++++++++++++++++++++++++++++++-
 monitor/hmp-cmds.c                |  1 +
 qapi/misc.json                    |  7 +++++++
 qapi/qom.json                     |  8 +++++++-
 qemu-options.hx                   |  7 ++++++-
 tests/unit/test-nested-aio-poll.c |  2 +-
 util/aio-posix.c                  | 17 +++++-----------
 util/aio-win32.c                  |  3 ++-
 util/async.c                      |  1 +
 11 files changed, 67 insertions(+), 18 deletions(-)

diff --git a/include/qemu/aio.h b/include/qemu/aio.h
index 6c77a190e9..50b8db2712 100644
--- a/include/qemu/aio.h
+++ b/include/qemu/aio.h
@@ -311,6 +311,7 @@ struct AioContext {
     int64_t poll_max_ns;    /* maximum polling time in nanoseconds */
     int64_t poll_grow;      /* polling time growth factor */
     int64_t poll_shrink;    /* polling time shrink factor */
+    int64_t poll_weight;    /* weight of current interval in calculation */
 
     /* AIO engine parameters */
     int64_t aio_max_batch;  /* maximum number of requests in a batch */
@@ -792,12 +793,13 @@ void aio_context_destroy(AioContext *ctx);
  * @max_ns: how long to busy poll for, in nanoseconds
  * @grow: polling time growth factor
  * @shrink: polling time shrink factor
+ * @weight: weight factor applied to the current polling interval
  *
  * Poll mode can be disabled by setting poll_max_ns to 0.
  */
 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
                                  int64_t grow, int64_t shrink,
-                                 Error **errp);
+                                 int64_t weight, Error **errp);
 
 /**
  * aio_context_set_aio_params:
diff --git a/include/system/iothread.h b/include/system/iothread.h
index e26d13c6c7..6ea57ed126 100644
--- a/include/system/iothread.h
+++ b/include/system/iothread.h
@@ -38,6 +38,7 @@ struct IOThread {
     int64_t poll_max_ns;
     int64_t poll_grow;
     int64_t poll_shrink;
+    int64_t poll_weight;
 };
 typedef struct IOThread IOThread;
 
diff --git a/iothread.c b/iothread.c
index caf68e0764..0389b8f7a8 100644
--- a/iothread.c
+++ b/iothread.c
@@ -32,8 +32,14 @@
  * workloads.
  */
 #define IOTHREAD_POLL_MAX_NS_DEFAULT 32768ULL
+#define IOTHREAD_POLL_GROW_DEFAULT 2ULL
+#define IOTHREAD_POLL_SHRINK_DEFAULT 2ULL
+#define IOTHREAD_POLL_WEIGHT_DEFAULT 3ULL
 #else
 #define IOTHREAD_POLL_MAX_NS_DEFAULT 0ULL
+#define IOTHREAD_POLL_GROW_DEFAULT 0ULL
+#define IOTHREAD_POLL_SHRINK_DEFAULT 0ULL
+#define IOTHREAD_POLL_WEIGHT_DEFAULT 0ULL
 #endif
 
 static void *iothread_run(void *opaque)
@@ -103,6 +109,10 @@ static void iothread_instance_init(Object *obj)
     IOThread *iothread = IOTHREAD(obj);
 
     iothread->poll_max_ns = IOTHREAD_POLL_MAX_NS_DEFAULT;
+    iothread->poll_grow = IOTHREAD_POLL_GROW_DEFAULT;
+    iothread->poll_shrink = IOTHREAD_POLL_SHRINK_DEFAULT;
+    iothread->poll_weight = IOTHREAD_POLL_WEIGHT_DEFAULT;
+
     iothread->thread_id = -1;
     qemu_sem_init(&iothread->init_done_sem, 0);
     /* By default, we don't run gcontext */
@@ -164,6 +174,7 @@ static void iothread_set_aio_context_params(EventLoopBase *base, Error **errp)
                                 iothread->poll_max_ns,
                                 iothread->poll_grow,
                                 iothread->poll_shrink,
+                                iothread->poll_weight,
                                 errp);
     if (*errp) {
         return;
@@ -233,6 +244,9 @@ static IOThreadParamInfo poll_grow_info = {
 static IOThreadParamInfo poll_shrink_info = {
     "poll-shrink", offsetof(IOThread, poll_shrink),
 };
+static IOThreadParamInfo poll_weight_info = {
+    "poll-weight", offsetof(IOThread, poll_weight),
+};
 
 static void iothread_get_param(Object *obj, Visitor *v,
         const char *name, IOThreadParamInfo *info, Error **errp)
@@ -260,7 +274,19 @@ static bool iothread_set_param(Object *obj, Visitor *v,
         return false;
     }
 
-    *field = value;
+    if (value == 0) {
+        if (info->offset == offsetof(IOThread, poll_grow)) {
+            *field = IOTHREAD_POLL_GROW_DEFAULT;
+        } else if (info->offset == offsetof(IOThread, poll_shrink)) {
+            *field = IOTHREAD_POLL_SHRINK_DEFAULT;
+        } else if (info->offset == offsetof(IOThread, poll_weight)) {
+            *field = IOTHREAD_POLL_WEIGHT_DEFAULT;
+        } else {
+            *field = value;
+        }
+    } else {
+        *field = value;
+    }
 
     return true;
 }
@@ -288,6 +314,7 @@ static void iothread_set_poll_param(Object *obj, Visitor *v,
                                     iothread->poll_max_ns,
                                     iothread->poll_grow,
                                     iothread->poll_shrink,
+                                    iothread->poll_weight,
                                     errp);
     }
 }
@@ -311,6 +338,10 @@ static void iothread_class_init(ObjectClass *klass, const void *class_data)
                               iothread_get_poll_param,
                               iothread_set_poll_param,
                               NULL, &poll_shrink_info);
+    object_class_property_add(klass, "poll-weight", "int",
+                              iothread_get_poll_param,
+                              iothread_set_poll_param,
+                              NULL, &poll_weight_info);
 }
 
 static const TypeInfo iothread_info = {
@@ -356,6 +387,7 @@ static int query_one_iothread(Object *object, void *opaque)
     info->poll_max_ns = iothread->poll_max_ns;
     info->poll_grow = iothread->poll_grow;
     info->poll_shrink = iothread->poll_shrink;
+    info->poll_weight = iothread->poll_weight;
     info->aio_max_batch = iothread->parent_obj.aio_max_batch;
 
     QAPI_LIST_APPEND(*tail, info);
diff --git a/monitor/hmp-cmds.c b/monitor/hmp-cmds.c
index bad034937a..75b6e7fa65 100644
--- a/monitor/hmp-cmds.c
+++ b/monitor/hmp-cmds.c
@@ -206,6 +206,7 @@ void hmp_info_iothreads(Monitor *mon, const QDict *qdict)
         monitor_printf(mon, "  poll-max-ns=%" PRId64 "\n", value->poll_max_ns);
         monitor_printf(mon, "  poll-grow=%" PRId64 "\n", value->poll_grow);
         monitor_printf(mon, "  poll-shrink=%" PRId64 "\n", value->poll_shrink);
+        monitor_printf(mon, "  poll-weight=%" PRId64 "\n", value->poll_weight);
         monitor_printf(mon, "  aio-max-batch=%" PRId64 "\n",
                        value->aio_max_batch);
     }
diff --git a/qapi/misc.json b/qapi/misc.json
index 28c641fe2f..39d17010bc 100644
--- a/qapi/misc.json
+++ b/qapi/misc.json
@@ -85,6 +85,12 @@
 # @poll-shrink: how many ns will be removed from polling time, 0 means
 #     that it's not configured (since 2.9)
 #
+# @poll-weight: the weight factor for adaptive polling.
+#     Determines how much the current event interval contributes to
+#     the next polling time calculation.  Valid values are 1 or
+#     greater.  0 selects the system default value which is current 3
+#     (since 10.2)
+#
 # @aio-max-batch: maximum number of requests in a batch for the AIO
 #     engine, 0 means that the engine will use its default (since 6.1)
 #
@@ -96,6 +102,7 @@
            'poll-max-ns': 'int',
            'poll-grow': 'int',
            'poll-shrink': 'int',
+           'poll-weight': 'int',
            'aio-max-batch': 'int' } }
 
 ##
diff --git a/qapi/qom.json b/qapi/qom.json
index c653248f85..feb80b6cfe 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -606,6 +606,11 @@
 #     algorithm detects it is spending too long polling without
 #     encountering events.  0 selects a default behaviour (default: 0)
 #
+# @poll-weight: the weight factor for adaptive polling.
+#     Determines how much the current event interval contributes to
+#     the next polling time calculation.  Valid values are 1 or
+#     greater.  If set to 0, the default value of 3 is used.
+#
 # The @aio-max-batch option is available since 6.1.
 #
 # Since: 2.0
@@ -614,7 +619,8 @@
   'base': 'EventLoopBaseProperties',
   'data': { '*poll-max-ns': 'int',
             '*poll-grow': 'int',
-            '*poll-shrink': 'int' } }
+            '*poll-shrink': 'int',
+            '*poll-weight': 'int' } }
 
 ##
 # @MainLoopProperties:
diff --git a/qemu-options.hx b/qemu-options.hx
index 69e5a874c1..8ddf6c8d36 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -6413,7 +6413,7 @@ SRST
 
             CN=laptop.example.com,O=Example Home,L=London,ST=London,C=GB
 
-    ``-object iothread,id=id,poll-max-ns=poll-max-ns,poll-grow=poll-grow,poll-shrink=poll-shrink,aio-max-batch=aio-max-batch``
+    ``-object iothread,id=id,poll-max-ns=poll-max-ns,poll-grow=poll-grow,poll-shrink=poll-shrink,poll-weight=poll-weight,aio-max-batch=aio-max-batch``
         Creates a dedicated event loop thread that devices can be
         assigned to. This is known as an IOThread. By default device
         emulation happens in vCPU threads or the main event loop thread.
@@ -6449,6 +6449,11 @@ SRST
         the polling time when the algorithm detects it is spending too
         long polling without encountering events.
 
+        The ``poll-weight`` parameter is the weight factor used in the
+        adaptive polling algorithm. It determines how much the most
+        recent event interval affects the calculation of the next
+        polling duration.
+
         The ``aio-max-batch`` parameter is the maximum number of requests
         in a batch for the AIO engine, 0 means that the engine will use
         its default.
diff --git a/tests/unit/test-nested-aio-poll.c b/tests/unit/test-nested-aio-poll.c
index 9ab1ad08a7..4c38f36fd4 100644
--- a/tests/unit/test-nested-aio-poll.c
+++ b/tests/unit/test-nested-aio-poll.c
@@ -81,7 +81,7 @@ static void test(void)
     qemu_set_current_aio_context(td.ctx);
 
     /* Enable polling */
-    aio_context_set_poll_params(td.ctx, 1000000, 2, 2, &error_abort);
+    aio_context_set_poll_params(td.ctx, 1000000, 2, 2, 3, &error_abort);
 
     /* Make the event notifier active (set) right away */
     event_notifier_init(&td.poll_notifier, 1);
diff --git a/util/aio-posix.c b/util/aio-posix.c
index 2b3522f2f9..13b7f94911 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -29,7 +29,6 @@
 
 /* Stop userspace polling on a handler if it isn't active for some time */
 #define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
-#define POLL_WEIGHT_SHIFT   (3)
 
 static void adjust_block_ns(AioContext *ctx, int64_t block_ns);
 static void grow_polling_time(AioContext *ctx, int64_t block_ns);
@@ -593,10 +592,6 @@ static void shrink_polling_time(AioContext *ctx, int64_t block_ns)
     int64_t old = ctx->poll_ns;
     int64_t shrink = ctx->poll_shrink;
 
-    if (shrink == 0) {
-        shrink = 2;
-    }
-
     if (block_ns < (ctx->poll_ns / shrink)) {
         ctx->poll_ns /= shrink;
     }
@@ -610,10 +605,6 @@ static void grow_polling_time(AioContext *ctx, int64_t block_ns)
     int64_t old = ctx->poll_ns;
     int64_t grow = ctx->poll_grow;
 
-    if (grow == 0) {
-        grow = 2;
-    }
-
     if (block_ns > ctx->poll_ns * grow) {
         ctx->poll_ns = block_ns;
     } else {
@@ -640,8 +631,8 @@ static void adjust_block_ns(AioContext *ctx, int64_t block_ns)
              * poll.ns to smooth out polling time adjustments.
              */
             node->poll.ns = node->poll.ns
-                ? (node->poll.ns - (node->poll.ns >> POLL_WEIGHT_SHIFT))
-                + (block_ns >> POLL_WEIGHT_SHIFT) : block_ns;
+                ? (node->poll.ns - (node->poll.ns >> ctx->poll_weight))
+                + (block_ns >> ctx->poll_weight) : block_ns;
 
             if (node->poll.ns > ctx->poll_max_ns) {
                 node->poll.ns = 0;
@@ -831,7 +822,8 @@ void aio_context_destroy(AioContext *ctx)
 }
 
 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
-                                 int64_t grow, int64_t shrink, Error **errp)
+                                 int64_t grow, int64_t shrink,
+                                 int64_t weight, Error **errp)
 {
     AioHandler *node;
 
@@ -848,6 +840,7 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
     ctx->poll_max_ns = max_ns;
     ctx->poll_grow = grow;
     ctx->poll_shrink = shrink;
+    ctx->poll_weight = weight;
     ctx->poll_ns = 0;
 
     aio_notify(ctx);
diff --git a/util/aio-win32.c b/util/aio-win32.c
index 6e6f699e4b..1985843233 100644
--- a/util/aio-win32.c
+++ b/util/aio-win32.c
@@ -429,7 +429,8 @@ void aio_context_destroy(AioContext *ctx)
 }
 
 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
-                                 int64_t grow, int64_t shrink, Error **errp)
+                                 int64_t grow, int64_t shrink,
+                                 int64_t weight, Error **errp)
 {
     if (max_ns) {
         error_setg(errp, "AioContext polling is not implemented on Windows");
diff --git a/util/async.c b/util/async.c
index 9d3627566f..741fcfd6a7 100644
--- a/util/async.c
+++ b/util/async.c
@@ -609,6 +609,7 @@ AioContext *aio_context_new(Error **errp)
     ctx->poll_ns = 0;
     ctx->poll_grow = 0;
     ctx->poll_shrink = 0;
+    ctx->poll_weight = 0;
 
     ctx->aio_max_batch = 0;
 
-- 
2.50.1

Re: [PATCH RFC v2 3/3] qapi/iothread: introduce poll-weight parameter for aio-poll

[Markus Armbruster]

Jaehoon Kim <jhkim@linux.ibm.com> writes:

> Introduce a configurable poll-weight parameter for adaptive polling
> in IOThread. This parameter replaces the hardcoded POLL_WEIGHT_SHIFT
> constant, allowing runtime control over how much the most recent
> event interval affects the next polling duration calculation.
>
> The poll-weight parameter uses a shift value where larger values
> decrease the weight of the current interval, enabling more gradual
> adjustments. When set to 0, a default value of 3 is used (meaning
> the current interval contributes approximately 1/8 to the weighted
> average).
>
> This patch also removes the hardcoded default values for poll-grow
> and poll-shrink parameters from the grow_polling_time() and
> shrink_polling_time() functions, as these defaults are now properly
> initialized in iothread.c during IOThread creation.
>
> Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>

[...]

> diff --git a/qapi/misc.json b/qapi/misc.json
> index 28c641fe2f..39d17010bc 100644
> --- a/qapi/misc.json
> +++ b/qapi/misc.json
> @@ -85,6 +85,12 @@

Note: this is IOThreadInfo, used only as return value of
query-iothreads.

>  # @poll-shrink: how many ns will be removed from polling time, 0 means
>  #     that it's not configured (since 2.9)
>  #
> +# @poll-weight: the weight factor for adaptive polling.
> +#     Determines how much the current event interval contributes to
> +#     the next polling time calculation.  Valid values are 1 or
> +#     greater.  0 selects the system default value which is current 3

Does query-iothreads actually return 0?  I'd expect it to return the
value that is actually used.

> +#     (since 10.2)

11.1 most likely.

> +#
>  # @aio-max-batch: maximum number of requests in a batch for the AIO
>  #     engine, 0 means that the engine will use its default (since 6.1)
>  #
> @@ -96,6 +102,7 @@
>             'poll-max-ns': 'int',
>             'poll-grow': 'int',
>             'poll-shrink': 'int',
> +           'poll-weight': 'int',
>             'aio-max-batch': 'int' } }
>  
>  ##
> diff --git a/qapi/qom.json b/qapi/qom.json
> index c653248f85..feb80b6cfe 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -606,6 +606,11 @@
>  #     algorithm detects it is spending too long polling without
>  #     encountering events.  0 selects a default behaviour (default: 0)
>  #
> +# @poll-weight: the weight factor for adaptive polling.
> +#     Determines how much the current event interval contributes to
> +#     the next polling time calculation.  Valid values are 1 or
> +#     greater.  If set to 0, the default value of 3 is used.

The commit message hints what the valid values mean, the doc comment
doesn't even that.  Do users need to know?

Code [*] below uses it like time >> poll_weight, where @time is int64_t.
poll_weight > 63 is undefined behavior, which is a no-no.  Please reject
such values.  poll_weight == 64 results in zero.  Is that useful?

Missing: (default: 0) (since 11.1)

> +#
>  # The @aio-max-batch option is available since 6.1.
>  #
>  # Since: 2.0
> @@ -614,7 +619,8 @@
>    'base': 'EventLoopBaseProperties',
>    'data': { '*poll-max-ns': 'int',
>              '*poll-grow': 'int',
> -            '*poll-shrink': 'int' } }
> +            '*poll-shrink': 'int',
> +            '*poll-weight': 'int' } }
>  
>  ##
>  # @MainLoopProperties:
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 69e5a874c1..8ddf6c8d36 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -6413,7 +6413,7 @@ SRST
>  
>              CN=laptop.example.com,O=Example Home,L=London,ST=London,C=GB
>  
> -    ``-object iothread,id=id,poll-max-ns=poll-max-ns,poll-grow=poll-grow,poll-shrink=poll-shrink,aio-max-batch=aio-max-batch``
> +    ``-object iothread,id=id,poll-max-ns=poll-max-ns,poll-grow=poll-grow,poll-shrink=poll-shrink,poll-weight=poll-weight,aio-max-batch=aio-max-batch``
>          Creates a dedicated event loop thread that devices can be
>          assigned to. This is known as an IOThread. By default device
>          emulation happens in vCPU threads or the main event loop thread.
> @@ -6449,6 +6449,11 @@ SRST
>          the polling time when the algorithm detects it is spending too
>          long polling without encountering events.
>  
> +        The ``poll-weight`` parameter is the weight factor used in the
> +        adaptive polling algorithm. It determines how much the most
> +        recent event interval affects the calculation of the next
> +        polling duration.
> +
>          The ``aio-max-batch`` parameter is the maximum number of requests
>          in a batch for the AIO engine, 0 means that the engine will use
>          its default.
> diff --git a/tests/unit/test-nested-aio-poll.c b/tests/unit/test-nested-aio-poll.c
> index 9ab1ad08a7..4c38f36fd4 100644
> --- a/tests/unit/test-nested-aio-poll.c
> +++ b/tests/unit/test-nested-aio-poll.c
> @@ -81,7 +81,7 @@ static void test(void)
>      qemu_set_current_aio_context(td.ctx);
>  
>      /* Enable polling */
> -    aio_context_set_poll_params(td.ctx, 1000000, 2, 2, &error_abort);
> +    aio_context_set_poll_params(td.ctx, 1000000, 2, 2, 3, &error_abort);
>  
>      /* Make the event notifier active (set) right away */
>      event_notifier_init(&td.poll_notifier, 1);
> diff --git a/util/aio-posix.c b/util/aio-posix.c
> index 2b3522f2f9..13b7f94911 100644
> --- a/util/aio-posix.c
> +++ b/util/aio-posix.c
> @@ -29,7 +29,6 @@
>  
>  /* Stop userspace polling on a handler if it isn't active for some time */
>  #define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
> -#define POLL_WEIGHT_SHIFT   (3)
>  
>  static void adjust_block_ns(AioContext *ctx, int64_t block_ns);
>  static void grow_polling_time(AioContext *ctx, int64_t block_ns);
> @@ -593,10 +592,6 @@ static void shrink_polling_time(AioContext *ctx, int64_t block_ns)
>      int64_t old = ctx->poll_ns;
>      int64_t shrink = ctx->poll_shrink;
>  
> -    if (shrink == 0) {
> -        shrink = 2;
> -    }
> -
>      if (block_ns < (ctx->poll_ns / shrink)) {
>          ctx->poll_ns /= shrink;
>      }
> @@ -610,10 +605,6 @@ static void grow_polling_time(AioContext *ctx, int64_t block_ns)
>      int64_t old = ctx->poll_ns;
>      int64_t grow = ctx->poll_grow;
>  
> -    if (grow == 0) {
> -        grow = 2;
> -    }
> -
>      if (block_ns > ctx->poll_ns * grow) {
>          ctx->poll_ns = block_ns;
>      } else {
> @@ -640,8 +631,8 @@ static void adjust_block_ns(AioContext *ctx, int64_t block_ns)
>               * poll.ns to smooth out polling time adjustments.
>               */
>              node->poll.ns = node->poll.ns
> -                ? (node->poll.ns - (node->poll.ns >> POLL_WEIGHT_SHIFT))
> -                + (block_ns >> POLL_WEIGHT_SHIFT) : block_ns;
> +                ? (node->poll.ns - (node->poll.ns >> ctx->poll_weight))
> +                + (block_ns >> ctx->poll_weight) : block_ns;

[*] This is the use of @poll-weight referred to above.

>  
>              if (node->poll.ns > ctx->poll_max_ns) {
>                  node->poll.ns = 0;
> @@ -831,7 +822,8 @@ void aio_context_destroy(AioContext *ctx)
>  }
>  
>  void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
> -                                 int64_t grow, int64_t shrink, Error **errp)
> +                                 int64_t grow, int64_t shrink,
> +                                 int64_t weight, Error **errp)
>  {
>      AioHandler *node;
>  
> @@ -848,6 +840,7 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
>      ctx->poll_max_ns = max_ns;
>      ctx->poll_grow = grow;
>      ctx->poll_shrink = shrink;
> +    ctx->poll_weight = weight;
>      ctx->poll_ns = 0;
>  
>      aio_notify(ctx);

[...]

Re: [PATCH RFC v2 3/3] qapi/iothread: introduce poll-weight parameter for aio-poll

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 1777 bytes --]

On Mon, Mar 23, 2026 at 08:54:51AM -0500, Jaehoon Kim wrote:
> diff --git a/qapi/misc.json b/qapi/misc.json
> index 28c641fe2f..39d17010bc 100644
> --- a/qapi/misc.json
> +++ b/qapi/misc.json
> @@ -85,6 +85,12 @@
>  # @poll-shrink: how many ns will be removed from polling time, 0 means
>  #     that it's not configured (since 2.9)
>  #
> +# @poll-weight: the weight factor for adaptive polling.
> +#     Determines how much the current event interval contributes to
> +#     the next polling time calculation.  Valid values are 1 or
> +#     greater.  0 selects the system default value which is current 3
> +#     (since 10.2)

QEMU 11.0 is already in hard freeze, so this patch will go into 11.1:

  (since 11.1)

> +#
>  # @aio-max-batch: maximum number of requests in a batch for the AIO
>  #     engine, 0 means that the engine will use its default (since 6.1)
>  #
> @@ -96,6 +102,7 @@
>             'poll-max-ns': 'int',
>             'poll-grow': 'int',
>             'poll-shrink': 'int',
> +           'poll-weight': 'int',
>             'aio-max-batch': 'int' } }
>  
>  ##
> diff --git a/qapi/qom.json b/qapi/qom.json
> index c653248f85..feb80b6cfe 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -606,6 +606,11 @@
>  #     algorithm detects it is spending too long polling without
>  #     encountering events.  0 selects a default behaviour (default: 0)
>  #
> +# @poll-weight: the weight factor for adaptive polling.
> +#     Determines how much the current event interval contributes to
> +#     the next polling time calculation.  Valid values are 1 or
> +#     greater.  If set to 0, the default value of 3 is used.

(since 11.1)

Other than that:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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Re: [PATCH RFC v2 3/3] qapi/iothread: introduce poll-weight parameter for aio-poll

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 975 bytes --]

On Mon, Mar 23, 2026 at 08:54:51AM -0500, Jaehoon Kim wrote:
> @@ -831,7 +822,8 @@ void aio_context_destroy(AioContext *ctx)
>  }
>  
>  void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
> -                                 int64_t grow, int64_t shrink, Error **errp)
> +                                 int64_t grow, int64_t shrink,
> +                                 int64_t weight, Error **errp)
>  {
>      AioHandler *node;
>  
> @@ -848,6 +840,7 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
>      ctx->poll_max_ns = max_ns;
>      ctx->poll_grow = grow;
>      ctx->poll_shrink = shrink;
> +    ctx->poll_weight = weight;
>      ctx->poll_ns = 0;
>  
>      aio_notify(ctx);

On second thought, now that the divide-by-0 protection has been removed
and these fields are assumed to hold a valid value when poll_max_ns > 0,
aio_context_set_poll_params() needs the same 0-protection as
iothread_set_param().

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Re: [PATCH RFC v2 1/3] aio-poll: avoid unnecessary polling time computation

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 752 bytes --]

On Mon, Mar 23, 2026 at 08:54:49AM -0500, Jaehoon Kim wrote:
> Nodes are no longer added to poll_aio_handlers when adaptive polling is
> disabled, preventing unnecessary try_poll_mode() calls. Additionally,
> aio_poll() skips try_poll_mode() when timeout is 0.

Skipping when timeout is 0 seems risky to me. VIRTIO devices disable
guest kicks when polling mode is started. When aio_poll(ctx,
blocking=false) is called, we will skip polling and
ctx->fdmon_ops->need_wait(ctx) won't detect an event either. aio_poll()
will return without noticing that the VIRTIO device's AioHandler is
ready.

Is skipping when timeout 0 necessary for performance or can it be
dropped from the patch?

Aside from this:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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Re: [PATCH RFC v2 2/3] aio-poll: refine iothread polling using weighted handler intervals

[Stefan Hajnoczi]

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On Mon, Mar 23, 2026 at 08:54:50AM -0500, Jaehoon Kim wrote:
> Refine adaptive polling in aio_poll by updating iothread polling
> duration based on weighted AioHandler event intervals.
> 
> Each AioHandler's poll.ns is updated using a weighted factor when an
> event occurs. Idle handlers accumulate block_ns until poll_max_ns and
> then reset to 0, preventing sporadically active handlers from
> unnecessarily prolonging iothread polling.
> 
> The iothread polling duration is set based on the largest poll.ns among
> active handlers. The shrink divider defaults to 2, matching the grow
> rate, to reduce frequent poll_ns resets for slow devices.
> 
> The default weight factor (POLL_WEIGHT_SHIFT=3, meaning the current
> interval contributes 12.5% to the weighted average) was selected based
> on extensive testing comparing QEMU 10.0.0 baseline vs poll-weight=2
> and poll-weight=3 across various workloads.
> 
> The table below shows a comparison between:
> -Host: RHEL 10.1 GA + qemu-10.0.0-14.el10_1, Guest: RHEL 9.6GA vs.
> -Host: RHEL 10.1 GA + qemu-10.0.0-14.el10_1 (w=2/w=3), Guest: RHEL 9.6GA
> for FIO FCP and FICON with 1 iothread and 8 iothreads.
> The values shown are the averages for numjobs 1, 4, and 8.
> 
> Summary of results (% change vs baseline):
> 
>                     | poll-weight=2      | poll-weight=3
> --------------------|--------------------|-----------------
> Throughput avg      | -2.4% (all tests)  | -2.2% (all tests)
> CPU consumption avg | -10.9% (all tests) | -9.4% (all tests)
> 
> Both weight=2 and weight=3 show significant CPU consumption reduction
> (~10%) compared to baseline, which addresses the CPU utilization
> regression observed in QEMU 10.0.0. The throughput impact is minimal
> for both (~2%).
> 
> Weight=3 is selected as the default because it provides slightly better
> throughput (-2.2% vs -2.4%) while still achieving substantial CPU
> savings (-9.4%). The difference between weight=2 and weight=3 is small,
> but weight=3 offers a better balance for general-purpose workloads.
> 
> Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
> ---
>  include/qemu/aio.h |   4 +-
>  util/aio-posix.c   | 135 +++++++++++++++++++++++++++++++--------------
>  util/async.c       |   1 +
>  3 files changed, 99 insertions(+), 41 deletions(-)
> 
> diff --git a/include/qemu/aio.h b/include/qemu/aio.h
> index 8cca2360d1..6c77a190e9 100644
> --- a/include/qemu/aio.h
> +++ b/include/qemu/aio.h
> @@ -195,7 +195,8 @@ struct BHListSlice {
>  typedef QSLIST_HEAD(, AioHandler) AioHandlerSList;
>  
>  typedef struct AioPolledEvent {
> -    int64_t ns;        /* current polling time in nanoseconds */
> +    bool has_event; /* Flag to indicate if an event has occurred */
> +    int64_t ns;     /* estimated block time in nanoseconds */
>  } AioPolledEvent;
>  
>  struct AioContext {
> @@ -306,6 +307,7 @@ struct AioContext {
>      int poll_disable_cnt;
>  
>      /* Polling mode parameters */
> +    int64_t poll_ns;        /* current polling time in nanoseconds */
>      int64_t poll_max_ns;    /* maximum polling time in nanoseconds */
>      int64_t poll_grow;      /* polling time growth factor */
>      int64_t poll_shrink;    /* polling time shrink factor */
> diff --git a/util/aio-posix.c b/util/aio-posix.c
> index b02beb0505..2b3522f2f9 100644
> --- a/util/aio-posix.c
> +++ b/util/aio-posix.c
> @@ -29,9 +29,11 @@
>  
>  /* Stop userspace polling on a handler if it isn't active for some time */
>  #define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
> +#define POLL_WEIGHT_SHIFT   (3)
>  
> -static void adjust_polling_time(AioContext *ctx, AioPolledEvent *poll,
> -                                int64_t block_ns);
> +static void adjust_block_ns(AioContext *ctx, int64_t block_ns);
> +static void grow_polling_time(AioContext *ctx, int64_t block_ns);
> +static void shrink_polling_time(AioContext *ctx, int64_t block_ns);
>  
>  bool aio_poll_disabled(AioContext *ctx)
>  {
> @@ -373,7 +375,7 @@ static bool aio_dispatch_ready_handlers(AioContext *ctx,
>           * add the handler to ctx->poll_aio_handlers.

This comment refers to adjusting the polling time. The code no longer
does this and the comment should be updated.

>           */
>          if (ctx->poll_max_ns && QLIST_IS_INSERTED(node, node_poll)) {
> -            adjust_polling_time(ctx, &node->poll, block_ns);

aio_dispatch_ready_handlers() no longer uses the block_ns argument. It
can be removed.

> +            node->poll.has_event = true;
>          }
>      }
>  
> @@ -560,18 +562,13 @@ static bool run_poll_handlers(AioContext *ctx, AioHandlerList *ready_list,
>  static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
>                            int64_t *timeout)
>  {
> -    AioHandler *node;
>      int64_t max_ns;
>  
>      if (QLIST_EMPTY_RCU(&ctx->poll_aio_handlers)) {
>          return false;
>      }
>  
> -    max_ns = 0;
> -    QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
> -        max_ns = MAX(max_ns, node->poll.ns);
> -    }
> -    max_ns = qemu_soonest_timeout(*timeout, max_ns);
> +    max_ns = qemu_soonest_timeout(*timeout, ctx->poll_ns);
>  
>      if (max_ns && !ctx->fdmon_ops->need_wait(ctx)) {
>          /*
> @@ -587,46 +584,98 @@ static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
>      return false;
>  }
>  
> -static void adjust_polling_time(AioContext *ctx, AioPolledEvent *poll,
> -                                int64_t block_ns)
> +static void shrink_polling_time(AioContext *ctx, int64_t block_ns)
>  {
> -    if (block_ns <= poll->ns) {
> -        /* This is the sweet spot, no adjustment needed */
> -    } else if (block_ns > ctx->poll_max_ns) {
> -        /* We'd have to poll for too long, poll less */
> -        int64_t old = poll->ns;
> -
> -        if (ctx->poll_shrink) {
> -            poll->ns /= ctx->poll_shrink;
> -        } else {
> -            poll->ns = 0;
> -        }
> +    /*
> +     * Reduce polling time if the block_ns is zero or
> +     * less than the current poll_ns.
> +     */
> +    int64_t old = ctx->poll_ns;
> +    int64_t shrink = ctx->poll_shrink;
>  
> -        trace_poll_shrink(ctx, old, poll->ns);
> -    } else if (poll->ns < ctx->poll_max_ns &&
> -               block_ns < ctx->poll_max_ns) {
> -        /* There is room to grow, poll longer */
> -        int64_t old = poll->ns;
> -        int64_t grow = ctx->poll_grow;
> +    if (shrink == 0) {
> +        shrink = 2;
> +    }
>  
> -        if (grow == 0) {
> -            grow = 2;
> -        }
> +    if (block_ns < (ctx->poll_ns / shrink)) {
> +        ctx->poll_ns /= shrink;
> +    }
>  
> -        if (poll->ns) {
> -            poll->ns *= grow;
> -        } else {
> -            poll->ns = 4000; /* start polling at 4 microseconds */
> -        }
> +    trace_poll_shrink(ctx, old, ctx->poll_ns);

This trace event should be inside if (block_ns < (ctx->poll_ns /
shrink)) like it was before this patch.

> +}
>  
> -        if (poll->ns > ctx->poll_max_ns) {
> -            poll->ns = ctx->poll_max_ns;
> -        }
> +static void grow_polling_time(AioContext *ctx, int64_t block_ns)
> +{
> +    /* There is room to grow, poll longer */
> +    int64_t old = ctx->poll_ns;
> +    int64_t grow = ctx->poll_grow;
>  
> -        trace_poll_grow(ctx, old, poll->ns);
> +    if (grow == 0) {
> +        grow = 2;
>      }
> +
> +    if (block_ns > ctx->poll_ns * grow) {
> +        ctx->poll_ns = block_ns;
> +    } else {
> +        ctx->poll_ns *= grow;
> +    }
> +
> +    if (ctx->poll_ns > ctx->poll_max_ns) {
> +        ctx->poll_ns = ctx->poll_max_ns;
> +    }
> +
> +    trace_poll_grow(ctx, old, ctx->poll_ns);

Same here.

>  }
>  
> +static void adjust_block_ns(AioContext *ctx, int64_t block_ns)
> +{
> +    AioHandler *node;
> +    int64_t adj_block_ns = -1;
> +
> +    QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
> +        if (node->poll.has_event) {

Did you consider unifying node->poll.has_event with
node->poll_idle_timeout, which is assigned now + POLL_IDLE_INTERVAL_NS
every time ->io_poll() detects an event?

For instance, rename node->poll_idle_timeout to
node->last_event_timestamp and assign now without adding
POLL_IDLE_INTERVAL_NS. Then use the field for both idle node removal and
adjust_block_ns() (pass in now).

> +            /*
> +             * Update poll.ns for the node with an event.
> +             * Uses a weighted average of the current block_ns and the previous
> +             * poll.ns to smooth out polling time adjustments.
> +             */
> +            node->poll.ns = node->poll.ns
> +                ? (node->poll.ns - (node->poll.ns >> POLL_WEIGHT_SHIFT))
> +                + (block_ns >> POLL_WEIGHT_SHIFT) : block_ns;
> +
> +            if (node->poll.ns > ctx->poll_max_ns) {
> +                node->poll.ns = 0;
> +            }

Previously:
-        if (poll->ns > ctx->poll_max_ns) {
-            poll->ns = ctx->poll_max_ns;
-        }

Was this causing excessive CPU consumption in your benchmarks?

Can you explain the rationale for zeroing the poll time? Aside from
reducing CPU consumption, it also reduces the chance that polling will
succeed and could therefore impact performance.

I'm asking about this because this patch makes several changes at once
and I'm not sure how the CPU usage and performance changes are
attributed to these multiple changes. I want to make sure the changes
merged are minimal and the best set - sometimes when multiple things are
changes at the same time, not all of them are beneficial.

> +            /*
> +             * To avoid excessive polling time increase, update adj_block_ns
> +             * for nodes with the event flag set to true
> +             */
> +            adj_block_ns = MAX(adj_block_ns, node->poll.ns);

adj_block_ns is not the blocking time, it's the maximum current poll
time across all nodes. It would be clearer to change the variable name.

> +            node->poll.has_event = false;
> +         } else {

4-space indentation should be used.

> +            /*
> +             * No event now, but was active before.
> +             * If it waits longer than poll_max_ns, poll.ns will stay 0
> +             * until the next event arrives.
> +             */
> +            if (node->poll.ns != 0) {
> +                node->poll.ns += block_ns;

Why is block_ns being added to an recently inactive node's polling time?
Here node->poll.ns no longer measures the weighted time until the
handler had an event.

If the goal is to get rid of inactive nodes, then maybe the idle handler
removal mechanism should be made more aggresive instead?

> +                if (node->poll.ns > ctx->poll_max_ns) {
> +                    node->poll.ns = 0;
> +                }
> +            }
> +        }
> +    }
> +
> +    if (adj_block_ns >= 0) {
> +        if (adj_block_ns > ctx->poll_ns) {
> +            grow_polling_time(ctx, adj_block_ns);
> +        } else {
> +            shrink_polling_time(ctx, adj_block_ns);
> +         }
> +     }
> + }
> +
>  bool aio_poll(AioContext *ctx, bool blocking)
>  {
>      AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
> @@ -723,6 +772,10 @@ bool aio_poll(AioContext *ctx, bool blocking)
>  
>      aio_free_deleted_handlers(ctx);
>  
> +    if (ctx->poll_max_ns) {
> +        adjust_block_ns(ctx, block_ns);
> +    }
> +
>      qemu_lockcnt_dec(&ctx->list_lock);
>  
>      progress |= timerlistgroup_run_timers(&ctx->tlg);
> @@ -784,6 +837,7 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
>  
>      qemu_lockcnt_inc(&ctx->list_lock);
>      QLIST_FOREACH(node, &ctx->aio_handlers, node) {
> +        node->poll.has_event = false;
>          node->poll.ns = 0;
>      }
>      qemu_lockcnt_dec(&ctx->list_lock);
> @@ -794,6 +848,7 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
>      ctx->poll_max_ns = max_ns;
>      ctx->poll_grow = grow;
>      ctx->poll_shrink = shrink;
> +    ctx->poll_ns = 0;
>  
>      aio_notify(ctx);
>  }
> diff --git a/util/async.c b/util/async.c
> index 80d6b01a8a..9d3627566f 100644
> --- a/util/async.c
> +++ b/util/async.c
> @@ -606,6 +606,7 @@ AioContext *aio_context_new(Error **errp)
>      timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
>  
>      ctx->poll_max_ns = 0;
> +    ctx->poll_ns = 0;
>      ctx->poll_grow = 0;
>      ctx->poll_shrink = 0;
>  
> -- 
> 2.50.1
> 

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