[PATCH 0/4] mm/damon: introduce DAMON_STAT for simple and practical access monitoring

[PATCH 0/4] mm/damon: introduce DAMON_STAT for simple and practical access monitoring

[SeongJae Park]

DAMON-based access monitoring is not simple due to required DAMON
control and results visualizations.  Introduce a static kernel module
for making it simple.  The module can be enabled without manual setup
and provides access pattern metrics that easy to fetch and understand
the practical access pattern information, namely estimated memory
bandwidth and memory idle time percentiles.

Background and Problems
=======================

DAMON can be used for monitoring data access patterns of the system and
workloads.  Specifically, users can start DAMON to monitor access events
on specific address space with fine controls including address ranges to
monitor and time intervals between samplings and aggregations.  The
resulting access information snapshot contains access frequency
(nr_accesses) and how long the frequency was kept (age) for each byte.

The monitoring usage is not simple and practical enough for production
usage.  Users should first start DAMON with a number of parameters, and
wait until DAMON's monitoring results capture a reasonable amount of the
time data (age).  In production, such manual start and wait is
impractical to capture useful information from a high number of machines
in a timely manner.

The monitoring result is also too detailed to be used on production
environments.  The raw results are hard to be aggregated and/or compared
for production environments having a large scale of time, space and
machines fleet.

Users have to implement and use their own automation of DAMON control
and results processing.  It is repetitive and challenging since there is
no good reference or guideline for such automation.

Solution: DAMON_STAT
====================

Implement such automation in kernel space as a static kernel module,
namely DAMON_STAT.  It can be enabled at build, boot, or run time via
its build configuration or module parameter.  It monitors the entire
physical address space with monitoring intervals that auto-tuned for a
reasonable amount of access observations and minimum overhead.  It
converts the raw monitoring results into simpler metrics that can easily
be aggregated and compared, namely estimated memory bandwidth and idle
time percentiles.  Refer to the commit messages of the second and the
third patches of this patch series for more details about the metrics.

Discussions
===========

The module aims to be useful on production environments constructed with
a large number of machines that run a long time.  The auto-tuned
monitoring intervals ensure a reasonable quality of the outputs.  The
auto-tuning also ensures its overhead be reasonable and low enough to be
enabled always on the production.  The simplified monitoring results
metrics can be useful for showing both coldness (idle time percentiles)
and hotness (memory bandwidth) of the system's access pattern.  We
expect the information can be useful for assessing system memory
utilization and inspiring optimizations or investigations on both kernel
and user space memory management logics for large scale fleets.

We hence expect the module is good enough to be just used in most
environments.  For special cases that require a custom access monitoring
automation, users will still benefit by using DAMON_STAT as a reference
or a guideline for their specialized automation.

Revision History
================

Changes from RFC
(https://lore.kernel.org/20250519164415.43935-1-sj@kernel.org)
- Add an admin-guide documentation
- Wordsmith commit messages
- Rebase to latest mm-new

SeongJae Park (4):
  mm/damon: introduce DAMON_STAT module
  mm/damon/stat: calculate and expose estimated memory bandwidth
  mm/damon/stat: calculate and expose idle time percentiles
  Docs/admin-guide/mm/damon: add DAMON_STAT usage document

 Documentation/admin-guide/mm/damon/index.rst |   1 +
 Documentation/admin-guide/mm/damon/stat.rst  |  69 ++++++
 mm/damon/Kconfig                             |  16 ++
 mm/damon/Makefile                            |   1 +
 mm/damon/stat.c                              | 245 +++++++++++++++++++
 5 files changed, 332 insertions(+)
 create mode 100644 Documentation/admin-guide/mm/damon/stat.rst
 create mode 100644 mm/damon/stat.c


base-commit: 90887f57d7a67917136e7c70d26fb3f2fcdc6f53
-- 
2.39.5

[PATCH 1/4] mm/damon: introduce DAMON_STAT module

[SeongJae Park]

To use DAMON for monitoring access patterns of the system, users should
manually start DAMON via DAMON sysfs ABI with a number of parameters for
specifying the monitoring target address space, address ranges, and
monitoring intervals.  After that, users should also wait until desired
amount of time data is captured into DAMON's monitoring results.  It is
bothersome and take a long time to be practical for access monitoring on
large fleet level production environments.

For access-aware system operations use cases like proactive cold memory
reclamation, similar problems existed.  We we solved those by
introducing dedicated static kernel modules such as DAMON_RECLAIM.

Implement such static kernel module for access monitoring, namely
DAMON_STAT.  It monitors the entire physical address space with
auto-tuned monitoring intervals.  The auto-tuning is set to capture 4 %
of observable access events in each snapshot while keeping the sampling
intervals 5 milliseconds in minimum and 10 seconds in maximum.  From
a few production environments, we confirmed this setup provides high
quality monitoring results with minimum overheads.  The module therefore
receives only one user input, whether to enable or disable it.  It can
be set on build or boot time via build configuration or kernel boot
command line.  It can also be overridden at runtime.

Note that this commit only implements the DAMON control part of the
module.  Users could get the monitoring results via
damon:damon_aggregated tracepoint, but that's of course not the
recommended way.  Following commits will implement convenient and
optimized ways for serving the monitoring results to users.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/Kconfig  |  16 ++++++
 mm/damon/Makefile |   1 +
 mm/damon/stat.c   | 138 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 155 insertions(+)
 create mode 100644 mm/damon/stat.c

diff --git a/mm/damon/Kconfig b/mm/damon/Kconfig
index 551745df011b..9f482e3adc67 100644
--- a/mm/damon/Kconfig
+++ b/mm/damon/Kconfig
@@ -95,4 +95,20 @@ config DAMON_LRU_SORT
 	  protect frequently accessed (hot) pages while rarely accessed (cold)
 	  pages reclaimed first under memory pressure.
 
+config DAMON_STAT
+	bool "Build data access monitoring stat (DAMON_STAT)"
+	depends on DAMON_PADDR
+	help
+	  This builds the DAMON-based access monitoring statistics subsystem.
+	  It runs DAMON and expose access monitoring results in simple stat
+	  metrics.
+
+config DAMON_STAT_ENABLED_DEFAULT
+	bool "Enable DAMON_STAT by default"
+	depends on DAMON_PADDR
+	default DAMON_STAT
+	help
+	  Whether to enable DAMON_STAT by default.  Users can disable it in
+	  boot or runtime using its 'enabled' parameter.
+
 endmenu
diff --git a/mm/damon/Makefile b/mm/damon/Makefile
index 8b49012ba8c3..d8d6bf5f8bff 100644
--- a/mm/damon/Makefile
+++ b/mm/damon/Makefile
@@ -6,3 +6,4 @@ obj-$(CONFIG_DAMON_PADDR)	+= ops-common.o paddr.o
 obj-$(CONFIG_DAMON_SYSFS)	+= sysfs-common.o sysfs-schemes.o sysfs.o
 obj-$(CONFIG_DAMON_RECLAIM)	+= modules-common.o reclaim.o
 obj-$(CONFIG_DAMON_LRU_SORT)	+= modules-common.o lru_sort.o
+obj-$(CONFIG_DAMON_STAT)	+= modules-common.o stat.o
diff --git a/mm/damon/stat.c b/mm/damon/stat.c
new file mode 100644
index 000000000000..852848ce844e
--- /dev/null
+++ b/mm/damon/stat.c
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Shows data access monitoring resutls in simple metrics.
+ */
+
+#define pr_fmt(fmt) "damon-stat: " fmt
+
+#include <linux/damon.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/sort.h>
+
+#ifdef MODULE_PARAM_PREFIX
+#undef MODULE_PARAM_PREFIX
+#endif
+#define MODULE_PARAM_PREFIX "damon_stat."
+
+static int damon_stat_enabled_store(
+		const char *val, const struct kernel_param *kp);
+
+static const struct kernel_param_ops enabled_param_ops = {
+	.set = damon_stat_enabled_store,
+	.get = param_get_bool,
+};
+
+static bool enabled __read_mostly = CONFIG_DAMON_STAT_ENABLED_DEFAULT;
+module_param_cb(enabled, &enabled_param_ops, &enabled, 0600);
+MODULE_PARM_DESC(enabled, "Enable of disable DAMON_STAT");
+
+static struct damon_ctx *damon_stat_context;
+
+static struct damon_ctx *damon_stat_build_ctx(void)
+{
+	struct damon_ctx *ctx;
+	struct damon_attrs attrs;
+	struct damon_target *target;
+	unsigned long start = 0, end = 0;
+
+	ctx = damon_new_ctx();
+	if (!ctx)
+		return NULL;
+	attrs = (struct damon_attrs) {
+		.sample_interval = 5 * USEC_PER_MSEC,
+		.aggr_interval = 100 * USEC_PER_MSEC,
+		.ops_update_interval = 60 * USEC_PER_MSEC * MSEC_PER_SEC,
+		.min_nr_regions = 10,
+		.max_nr_regions = 1000,
+	};
+	/*
+	 * auto-tune sampling and aggregation interval aiming 4% DAMON-observed
+	 * accesses ratio, keeping sampling interval in [5ms, 10s] range.
+	 */
+	attrs.intervals_goal = (struct damon_intervals_goal) {
+		.access_bp = 400, .aggrs = 3,
+		.min_sample_us = 5000, .max_sample_us = 10000000,
+	};
+	if (damon_set_attrs(ctx, &attrs))
+		goto free_out;
+
+	/*
+	 * auto-tune sampling and aggregation interval aiming 4% DAMON-observed
+	 * accesses ratio, keeping sampling interval in [5ms, 10s] range.
+	 */
+	ctx->attrs.intervals_goal = (struct damon_intervals_goal) {
+		.access_bp = 400, .aggrs = 3,
+		.min_sample_us = 5000, .max_sample_us = 10000000,
+	};
+	if (damon_select_ops(ctx, DAMON_OPS_PADDR))
+		goto free_out;
+
+	target = damon_new_target();
+	if (!target)
+		goto free_out;
+	damon_add_target(ctx, target);
+	if (damon_set_region_biggest_system_ram_default(target, &start, &end))
+		goto free_out;
+	return ctx;
+free_out:
+	damon_destroy_ctx(ctx);
+	return NULL;
+}
+
+static int damon_stat_start(void)
+{
+	damon_stat_context = damon_stat_build_ctx();
+	if (!damon_stat_context)
+		return -ENOMEM;
+	return damon_start(&damon_stat_context, 1, true);
+}
+
+static void damon_stat_stop(void)
+{
+	damon_stop(&damon_stat_context, 1);
+	damon_destroy_ctx(damon_stat_context);
+}
+
+static bool damon_stat_init_called;
+
+static int damon_stat_enabled_store(
+		const char *val, const struct kernel_param *kp)
+{
+	bool is_enabled = enabled;
+	int err;
+
+	err = kstrtobool(val, &enabled);
+	if (err)
+		return err;
+
+	if (is_enabled == enabled)
+		return 0;
+
+	if (!damon_stat_init_called)
+		/*
+		 * probably called from command line parsing (parse_args()).
+		 * Cannot call damon_new_ctx().  Let damon_stat_init() handle.
+		 */
+		return 0;
+
+	if (enabled)
+		return damon_stat_start();
+	damon_stat_stop();
+	return 0;
+}
+
+static int __init damon_stat_init(void)
+{
+	int err = 0;
+
+	damon_stat_init_called = true;
+
+	/* probably set via command line */
+	if (enabled)
+		err = damon_stat_start();
+	return err;
+}
+
+module_init(damon_stat_init);
-- 
2.39.5

[PATCH 2/4] mm/damon/stat: calculate and expose estimated memory bandwidth

[SeongJae Park]

The raw form of DAMON's monitoring results captures many details of the
information.  However, not every bit of the information is always
required for understanding practical access patterns.  Especially on
real world production systems of high scale time and size, the raw form
is difficult to be aggregated and compared.

Convert the raw monitoring results into a single number metric, namely
estimated memory bandwidth and expose it to users as a read-only
DAMON_STAT parameter.  The metric represents access intensiveness
(hotness) of the system.  It can easily be aggregated and compared for
high level understanding of the access pattern on large systems.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/stat.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index 852848ce844e..f9ae44db265b 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -28,8 +28,42 @@ static bool enabled __read_mostly = CONFIG_DAMON_STAT_ENABLED_DEFAULT;
 module_param_cb(enabled, &enabled_param_ops, &enabled, 0600);
 MODULE_PARM_DESC(enabled, "Enable of disable DAMON_STAT");
 
+static unsigned long estimated_memory_bandwidth __read_mostly;
+module_param(estimated_memory_bandwidth, ulong, 0400);
+MODULE_PARM_DESC(estimated_memory_bandwidth,
+		"Estimated memory bandwidth usage in bytes per second");
+
 static struct damon_ctx *damon_stat_context;
 
+static void damon_stat_set_estimated_memory_bandwidth(struct damon_ctx *c)
+{
+	struct damon_target *t;
+	struct damon_region *r;
+	unsigned long access_bytes = 0;
+
+	damon_for_each_target(t, c) {
+		damon_for_each_region(r, t)
+			access_bytes += (r->ar.end - r->ar.start) *
+				r->nr_accesses;
+	}
+	estimated_memory_bandwidth = access_bytes * USEC_PER_MSEC *
+		MSEC_PER_SEC / c->attrs.aggr_interval;
+}
+
+static int damon_stat_after_aggregation(struct damon_ctx *c)
+{
+	static unsigned long last_refresh_jiffies;
+
+	/* avoid unnecessarily frequent stat update */
+	if (time_before_eq(jiffies, last_refresh_jiffies +
+				msecs_to_jiffies(5 * MSEC_PER_SEC)))
+		return 0;
+	last_refresh_jiffies = jiffies;
+
+	damon_stat_set_estimated_memory_bandwidth(c);
+	return 0;
+}
+
 static struct damon_ctx *damon_stat_build_ctx(void)
 {
 	struct damon_ctx *ctx;
@@ -75,6 +109,7 @@ static struct damon_ctx *damon_stat_build_ctx(void)
 	damon_add_target(ctx, target);
 	if (damon_set_region_biggest_system_ram_default(target, &start, &end))
 		goto free_out;
+	ctx->callback.after_aggregation = damon_stat_after_aggregation;
 	return ctx;
 free_out:
 	damon_destroy_ctx(ctx);
-- 
2.39.5

[PATCH 3/4] mm/damon/stat: calculate and expose idle time percentiles

[SeongJae Park]

Knowing how much memory is how cold can be useful for understanding
coldness and utilization efficiency of memory.  The raw form of DAMON's
monitoring results has the information.  Convert the raw results into
the per-byte idle time distributions and expose it as percentiles metric
to users, as a read-only DAMON_STAT parameter.

In detail, the metrics are calculated as follows.  First, DAMON's
per-region access frequency and age information is converted into
per-byte idle time.  If access frequency of a region is higher than
zero, every byte of the region has zero idle time.  If the access
frequency of a region is zero, every byte of the region has idle time as
the age of the region.  Then the logic sorts the per-byte idle times and
provides the value at 0/100, 1/100, ..., 99/100 and 100/100 location of
the sorted array.

The metric can be easily aggregated and compared on large scale
production systems.  For example, if an average of 75-th percentile idle
time  of machines that collected on similar time is two minutes, it
means the system's 25 percent memory is not accessed at all for two
minutes or more on average.  If a workload considers two minutes as unit
work time, we can conclude its working set size is only 75 percent of
the memory.  If the system utilizes proactive reclamation and it
supports coldness-based thresholds like DAMON_RECLAIM, the idle time
percentiles can be used to find a more safe or aggressive coldness
threshold for aimed memory saving.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/stat.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index f9ae44db265b..7ef13ea22221 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -33,6 +33,11 @@ module_param(estimated_memory_bandwidth, ulong, 0400);
 MODULE_PARM_DESC(estimated_memory_bandwidth,
 		"Estimated memory bandwidth usage in bytes per second");
 
+static unsigned long memory_idle_ms_percentiles[101] __read_mostly = {0,};
+module_param_array(memory_idle_ms_percentiles, ulong, NULL, 0400);
+MODULE_PARM_DESC(memory_idle_ms_percentiles,
+		"Memory idle time percentiles in milliseconds");
+
 static struct damon_ctx *damon_stat_context;
 
 static void damon_stat_set_estimated_memory_bandwidth(struct damon_ctx *c)
@@ -50,6 +55,72 @@ static void damon_stat_set_estimated_memory_bandwidth(struct damon_ctx *c)
 		MSEC_PER_SEC / c->attrs.aggr_interval;
 }
 
+static unsigned int damon_stat_idletime(const struct damon_region *r)
+{
+	if (r->nr_accesses)
+		return 0;
+	return r->age + 1;
+}
+
+static int damon_stat_cmp_regions(const void *a, const void *b)
+{
+	const struct damon_region *ra = *(const struct damon_region **)a;
+	const struct damon_region *rb = *(const struct damon_region **)b;
+
+	return damon_stat_idletime(ra) - damon_stat_idletime(rb);
+}
+
+static int damon_stat_sort_regions(struct damon_ctx *c,
+		struct damon_region ***sorted_ptr, int *nr_regions_ptr,
+		unsigned long *total_sz_ptr)
+{
+	struct damon_target *t;
+	struct damon_region *r;
+	struct damon_region **region_pointers;
+	unsigned int nr_regions = 0;
+	unsigned long total_sz = 0;
+
+	damon_for_each_target(t, c) {
+		/* there is only one target */
+		region_pointers = kmalloc_array(damon_nr_regions(t),
+				sizeof(*region_pointers), GFP_KERNEL);
+		if (!region_pointers)
+			return -ENOMEM;
+		damon_for_each_region(r, t) {
+			region_pointers[nr_regions++] = r;
+			total_sz += r->ar.end - r->ar.start;
+		}
+	}
+	sort(region_pointers, nr_regions, sizeof(*region_pointers),
+			damon_stat_cmp_regions, NULL);
+	*sorted_ptr = region_pointers;
+	*nr_regions_ptr = nr_regions;
+	*total_sz_ptr = total_sz;
+	return 0;
+}
+
+static void damon_stat_set_idletime_percentiles(struct damon_ctx *c)
+{
+	struct damon_region **sorted_regions, *region;
+	int nr_regions;
+	unsigned long total_sz, accounted_bytes = 0;
+	int err, i, next_percentile = 0;
+
+	err = damon_stat_sort_regions(c, &sorted_regions, &nr_regions,
+			&total_sz);
+	if (err)
+		return;
+	for (i = 0; i < nr_regions; i++) {
+		region = sorted_regions[i];
+		accounted_bytes += region->ar.end - region->ar.start;
+		while (next_percentile <= accounted_bytes * 100 / total_sz)
+			memory_idle_ms_percentiles[next_percentile++] =
+				damon_stat_idletime(region) *
+				c->attrs.aggr_interval / USEC_PER_MSEC;
+	}
+	kfree(sorted_regions);
+}
+
 static int damon_stat_after_aggregation(struct damon_ctx *c)
 {
 	static unsigned long last_refresh_jiffies;
@@ -61,6 +132,7 @@ static int damon_stat_after_aggregation(struct damon_ctx *c)
 	last_refresh_jiffies = jiffies;
 
 	damon_stat_set_estimated_memory_bandwidth(c);
+	damon_stat_set_idletime_percentiles(c);
 	return 0;
 }
 
-- 
2.39.5

[PATCH 4/4] Docs/admin-guide/mm/damon: add DAMON_STAT usage document

[SeongJae Park]

Document DAMON_STAT usage and add a link to it on DAMON admin-guide
page.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/admin-guide/mm/damon/index.rst |  1 +
 Documentation/admin-guide/mm/damon/stat.rst  | 69 ++++++++++++++++++++
 2 files changed, 70 insertions(+)
 create mode 100644 Documentation/admin-guide/mm/damon/stat.rst

diff --git a/Documentation/admin-guide/mm/damon/index.rst b/Documentation/admin-guide/mm/damon/index.rst
index bc7e976120e0..3ce3164480c7 100644
--- a/Documentation/admin-guide/mm/damon/index.rst
+++ b/Documentation/admin-guide/mm/damon/index.rst
@@ -14,3 +14,4 @@ access monitoring and access-aware system operations.
    usage
    reclaim
    lru_sort
+   stat
diff --git a/Documentation/admin-guide/mm/damon/stat.rst b/Documentation/admin-guide/mm/damon/stat.rst
new file mode 100644
index 000000000000..4c517c2c219a
--- /dev/null
+++ b/Documentation/admin-guide/mm/damon/stat.rst
@@ -0,0 +1,69 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+Data Access Monitoring Results Stat
+===================================
+
+Data Access Monitoring Results Stat (DAMON_STAT) is a static kernel module that
+is aimed to be used for simple access pattern monitoring.  It monitors accesses
+on the system's entire physical memory using DAMON, and provides simplified
+access monitoring results statistics, namely idle time percentiles and
+estimated memory bandwidth.
+
+Monitoring Accuracy and Overhead
+================================
+
+DAMON_STAT uses monitoring intervals :ref:`auto-tuning
+<damon_design_monitoring_intervals_autotuning>` to make its accuracy high and
+overhead minimum.  It auto-tunes the intervals aiming 4 % of observable access
+events to be captured in each snapshot, while limiting the resulting sampling
+events to be 5 milliseconds in minimum and 10 seconds in maximum.  On a few
+production server systems, it resulted in consuming only 0.x % single CPU time,
+while capturing reasonable quality of access patterns.
+
+Interface: Module Parameters
+============================
+
+To use this feature, you should first ensure your system is running on a kernel
+that is built with ``CONFIG_DAMON_STAT=y``.  The feature can be enabled by
+default at build time, by setting ``CONFIG_DAMON_STAT_ENABLED_DEFAULT`` true.
+
+To let sysadmins enable or disable it at boot and/or runtime, and read the
+monitoring results, DAMON_STAT provides module parameters.  Following
+sections are descriptions of the parameters.
+
+enabled
+-------
+
+Enable or disable DAMON_STAT.
+
+You can enable DAMON_STAT by setting the value of this parameter as ``Y``.
+Setting it as ``N`` disables DAMON_STAT.  The default value is set by
+``CONFIG_DAMON_STAT_ENABLED_DEFAULT`` build config option.
+
+estimated_memory_bandwidth
+--------------------------
+
+Estimated memory bandwidth consumption (bytes per second) of the system.
+
+DAMON_STAT reads observed access events on the current DAMON results snapshot
+and converts it to memory bandwidth consumption estimation in bytes per second.
+The resulting metric is exposed to user via this read-only parameter.  Because
+DAMON uses sampling, this is only an estimation of the access intensity rather
+than accurate memory bandwidth.
+
+memory_idle_ms_percentiles
+--------------------------
+
+Per-byte idle time (milliseconds) percentiles of the system.
+
+DAMON_STAT calculates how long each byte of the memory was not accessed until
+now (idle time), based on the current DAMON results snapshot.  If DAMON found a
+region of access frequency (nr_accesses) larger than zero, every byte of the
+region gets zero idle time.  If a region has zero access frequency
+(nr_accesses), how long the region was keeping the zero access frequency (age)
+becomes the idle time of every byte of the region.  Then, DAMON_STAT exposes
+the percentiles of the idle time values via this read-only parameter.  Reading
+the parameter returns 101 idle time values in milliseconds, separated by comma.
+Each value represents 0-th, 1st, 2nd, 3rd, ..., 99th and 100th percentile idle
+times.
-- 
2.39.5

Re: [PATCH 0/4] mm/damon: introduce DAMON_STAT for simple and practical access monitoring

[Honggyu Kim]

Hi SeongJae,

Thanks for your work.

On 5/27/2025 6:09 AM, SeongJae Park wrote:
> DAMON-based access monitoring is not simple due to required DAMON
> control and results visualizations.  Introduce a static kernel module
> for making it simple.  The module can be enabled without manual setup
> and provides access pattern metrics that easy to fetch and understand
> the practical access pattern information, namely estimated memory
> bandwidth and memory idle time percentiles.
> 
> Background and Problems
> =======================
> 
> DAMON can be used for monitoring data access patterns of the system and
> workloads.  Specifically, users can start DAMON to monitor access events
> on specific address space with fine controls including address ranges to
> monitor and time intervals between samplings and aggregations.  The
> resulting access information snapshot contains access frequency
> (nr_accesses) and how long the frequency was kept (age) for each byte.
> 
> The monitoring usage is not simple and practical enough for production
> usage.  Users should first start DAMON with a number of parameters, and
> wait until DAMON's monitoring results capture a reasonable amount of the
> time data (age).  In production, such manual start and wait is
> impractical to capture useful information from a high number of machines
> in a timely manner.
> 
> The monitoring result is also too detailed to be used on production
> environments.  The raw results are hard to be aggregated and/or compared
> for production environments having a large scale of time, space and
> machines fleet.
> 
> Users have to implement and use their own automation of DAMON control
> and results processing.  It is repetitive and challenging since there is
> no good reference or guideline for such automation.
> 
> Solution: DAMON_STAT
> ====================
> 
> Implement such automation in kernel space as a static kernel module,
> namely DAMON_STAT.  It can be enabled at build, boot, or run time via
> its build configuration or module parameter.  It monitors the entire
> physical address space with monitoring intervals that auto-tuned for a
> reasonable amount of access observations and minimum overhead.  It
> converts the raw monitoring results into simpler metrics that can easily
> be aggregated and compared, namely estimated memory bandwidth and idle
> time percentiles.  Refer to the commit messages of the second and the
> third patches of this patch series for more details about the metrics.

I see the description looks good but it'd be useful if you could share some
execution commands and expected output examples that some newbies can get better
ideas.

I honestly do not have a clear idea how I can use this kind of static kernel
modules as general users although I have developed some features of DAMON.

So could you please help?

Thanks,
Honggyu

> 
> Discussions
> ===========
> 
> The module aims to be useful on production environments constructed with
> a large number of machines that run a long time.  The auto-tuned
> monitoring intervals ensure a reasonable quality of the outputs.  The
> auto-tuning also ensures its overhead be reasonable and low enough to be
> enabled always on the production.  The simplified monitoring results
> metrics can be useful for showing both coldness (idle time percentiles)
> and hotness (memory bandwidth) of the system's access pattern.  We
> expect the information can be useful for assessing system memory
> utilization and inspiring optimizations or investigations on both kernel
> and user space memory management logics for large scale fleets.
> 
> We hence expect the module is good enough to be just used in most
> environments.  For special cases that require a custom access monitoring
> automation, users will still benefit by using DAMON_STAT as a reference
> or a guideline for their specialized automation.
> 
> Revision History
> ================
> 
> Changes from RFC
> (https://lore.kernel.org/20250519164415.43935-1-sj@kernel.org)
> - Add an admin-guide documentation
> - Wordsmith commit messages
> - Rebase to latest mm-new
> 
> SeongJae Park (4):
>    mm/damon: introduce DAMON_STAT module
>    mm/damon/stat: calculate and expose estimated memory bandwidth
>    mm/damon/stat: calculate and expose idle time percentiles
>    Docs/admin-guide/mm/damon: add DAMON_STAT usage document
> 
>   Documentation/admin-guide/mm/damon/index.rst |   1 +
>   Documentation/admin-guide/mm/damon/stat.rst  |  69 ++++++
>   mm/damon/Kconfig                             |  16 ++
>   mm/damon/Makefile                            |   1 +
>   mm/damon/stat.c                              | 245 +++++++++++++++++++
>   5 files changed, 332 insertions(+)
>   create mode 100644 Documentation/admin-guide/mm/damon/stat.rst
>   create mode 100644 mm/damon/stat.c
> 
> 
> base-commit: 90887f57d7a67917136e7c70d26fb3f2fcdc6f53

Re: [PATCH 0/4] mm/damon: introduce DAMON_STAT for simple and practical access monitoring

[SeongJae Park]

Hi Honggyu,

On Thu, 29 May 2025 16:21:39 +0900 Honggyu Kim <honggyu.kim@sk.com> wrote:

> Hi SeongJae,
> 
> Thanks for your work.
> 
> On 5/27/2025 6:09 AM, SeongJae Park wrote:
> > DAMON-based access monitoring is not simple due to required DAMON
> > control and results visualizations.  Introduce a static kernel module
> > for making it simple.  The module can be enabled without manual setup
> > and provides access pattern metrics that easy to fetch and understand
> > the practical access pattern information, namely estimated memory
> > bandwidth and memory idle time percentiles.
> > 
> > Background and Problems
> > =======================
> > 
> > DAMON can be used for monitoring data access patterns of the system and
> > workloads.  Specifically, users can start DAMON to monitor access events
> > on specific address space with fine controls including address ranges to
> > monitor and time intervals between samplings and aggregations.  The
> > resulting access information snapshot contains access frequency
> > (nr_accesses) and how long the frequency was kept (age) for each byte.
> > 
> > The monitoring usage is not simple and practical enough for production
> > usage.  Users should first start DAMON with a number of parameters, and
> > wait until DAMON's monitoring results capture a reasonable amount of the
> > time data (age).  In production, such manual start and wait is
> > impractical to capture useful information from a high number of machines
> > in a timely manner.
> > 
> > The monitoring result is also too detailed to be used on production
> > environments.  The raw results are hard to be aggregated and/or compared
> > for production environments having a large scale of time, space and
> > machines fleet.
> > 
> > Users have to implement and use their own automation of DAMON control
> > and results processing.  It is repetitive and challenging since there is
> > no good reference or guideline for such automation.
> > 
> > Solution: DAMON_STAT
> > ====================
> > 
> > Implement such automation in kernel space as a static kernel module,
> > namely DAMON_STAT.  It can be enabled at build, boot, or run time via
> > its build configuration or module parameter.  It monitors the entire
> > physical address space with monitoring intervals that auto-tuned for a
> > reasonable amount of access observations and minimum overhead.  It
> > converts the raw monitoring results into simpler metrics that can easily
> > be aggregated and compared, namely estimated memory bandwidth and idle
> > time percentiles.  Refer to the commit messages of the second and the
> > third patches of this patch series for more details about the metrics.
> 
> I see the description looks good but it'd be useful if you could share some
> execution commands and expected output examples that some newbies can get better
> ideas.

Thank you for the feedback.  In my humble opinion, nevertheless, this is what
the fourth patch of this series is aiming to provide.  And thanks to your
comment, I now realize I forgot mentioning this on the above comment.

Of course we can add execution commands and output example here, but without
understanding of the metrics, it would just look like mysterious numbers.
Meanhile, user interface of the module is simply module parameters that should
be familiar to most kernel users, so I don't think it necessarily deserves
example commands.

> 
> I honestly do not have a clear idea how I can use this kind of static kernel
> modules as general users although I have developed some features of DAMON.
> 
> So could you please help?

I agree this cover letter is not enough for getting clear idea of how readers
can use this feature.  Thank you for letting me realize this.  But I think this
cover letter might be better to keep the brevity, as long as it points where
readers can get necessary details.  What do you think about revising this cover
letter to

1. point the fourth patch for usage of DAMON_STAT, and
2. elaborate the fact that readers are recommended to read the metric
   definitions and usages to get more clear idea of DAMON_STAT?


Thanks,
SJ

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