[Qemu-devel] [PATCH 2/2] qemu-timer: change unix timer to dynticks

[Paolo Bonzini <pbonzini@redhat.com>]

A timer that wakes up every millisecond puts a lot of stress on the
iothread.  The large amount of IPIs causes very high context switch
activity, making emulation slow and the UI unusable.  This is by the
way the same reason why the Windows timers were switched to dynticks.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Alexander Graf <agraf@suse.de>
---
 qemu-timer.c |   40 ++++++++++++++++++++++++++++------------
 1 files changed, 28 insertions(+), 12 deletions(-)

diff --git a/qemu-timer.c b/qemu-timer.c
index 4141b6e..5d78cc4 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -227,6 +227,7 @@ static void win32_rearm_timer(struct qemu_alarm_timer *t);
 
 static int unix_start_timer(struct qemu_alarm_timer *t);
 static void unix_stop_timer(struct qemu_alarm_timer *t);
+static void unix_rearm_timer(struct qemu_alarm_timer *t);
 
 #ifdef __linux__
 
@@ -309,7 +310,7 @@ static struct qemu_alarm_timer alarm_timers[] = {
     /* ...otherwise try RTC */
     {"rtc", rtc_start_timer, rtc_stop_timer, NULL},
 #endif
-    {"unix", unix_start_timer, unix_stop_timer, NULL},
+    {"unix", unix_start_timer, unix_stop_timer, unix_rearm_timer},
 #else
     {"mmtimer", mm_start_timer, mm_stop_timer, NULL},
     {"mmtimer2", mm_start_timer, mm_stop_timer, mm_rearm_timer},
@@ -1010,8 +1011,6 @@ static void dynticks_rearm_timer(struct qemu_alarm_timer *t)
 static int unix_start_timer(struct qemu_alarm_timer *t)
 {
     struct sigaction act;
-    struct itimerval itv;
-    int err;
 
     /* timer signal */
     sigfillset(&act.sa_mask);
@@ -1019,18 +1018,35 @@ static int unix_start_timer(struct qemu_alarm_timer *t)
     act.sa_handler = host_alarm_handler;
 
     sigaction(SIGALRM, &act, NULL);
+    return 0;
+}
 
-    itv.it_interval.tv_sec = 0;
-    /* for i386 kernel 2.6 to get 1 ms */
-    itv.it_interval.tv_usec = 999;
-    itv.it_value.tv_sec = 0;
-    itv.it_value.tv_usec = 10 * 1000;
+static void unix_rearm_timer(struct qemu_alarm_timer *t)
+{
+    struct itimerval itv;
+    int64_t nearest_delta_ns = INT64_MAX;
+    int err;
 
-    err = setitimer(ITIMER_REAL, &itv, NULL);
-    if (err)
-        return -1;
+    assert(alarm_has_dynticks(t));
+    if (!active_timers[QEMU_CLOCK_REALTIME] &&
+        !active_timers[QEMU_CLOCK_VIRTUAL] &&
+        !active_timers[QEMU_CLOCK_HOST])
+        return;
 
-    return 0;
+    nearest_delta_ns = qemu_next_alarm_deadline();
+    if (nearest_delta_ns < MIN_TIMER_REARM_NS)
+        nearest_delta_ns = MIN_TIMER_REARM_NS;
+
+    itv.it_interval.tv_sec = 0;
+    itv.it_interval.tv_usec = 0; /* 0 for one-shot timer */
+    itv.it_value.tv_sec =  nearest_delta_ns / 1000000000;
+    itv.it_value.tv_usec = (nearest_delta_ns % 1000000000) / 1000;
+    err = setitimer(ITIMER_REAL, &itv, NULL);
+    if (err) {
+        perror("setitimer");
+        fprintf(stderr, "Internal timer error: aborting\n");
+        exit(1);
+    }
 }
 
 static void unix_stop_timer(struct qemu_alarm_timer *t)
-- 
1.7.4.4