[Qemu-devel] [PATCH] Reduce redundant timer rearming

[Anders]

The dynticks code in qemu sets and gets timers very often. These
are the system calls (strace -c) of qemu/kvm running an idle Linux 
kernel at 250Hz for 10 seconds:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 99.67    0.014391           1     23248      3187 ioctl
  0.33    0.000047           0     43643           clock_gettime
  0.00    0.000000           0      6375           gettimeofday
  0.00    0.000000           0      6708           select
  0.00    0.000000           0      3187           rt_sigaction
  0.00    0.000000           0      9562      6375 rt_sigtimedwait
  0.00    0.000000           0     10311           timer_settime
  0.00    0.000000           0     12750           timer_gettime
------ ----------- ----------- --------- --------- ----------------
100.00    0.014438                115784      9562 total


The qemu_rearm_alarm_timer() function looks at vm_clock as well as 
rt_clock timers, but is called from qemu_run_timers() which looks at 
just one queue.

When an rt_clock timer has expired, the vm_clock iteration will rearm
with MIN_TIMER_REARM_US. This is not needed, since the timer in question 
will be removed right away when qemu_run_timers() is run on the rt_clock 
queue.

Moving the rearm call to after the two calls of qemu_run_timers() helps 
a lot:


% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 99.74    0.012590           1     22766      2706 ioctl
  0.26    0.000033           0     27044           clock_gettime
  0.00    0.000000           0      5413           gettimeofday
  0.00    0.000000           0      5745           select
  0.00    0.000000           0      2706           rt_sigaction
  0.00    0.000000           0      8119      5413 rt_sigtimedwait
  0.00    0.000000           0      5215           timer_settime
  0.00    0.000000           0      5413           timer_gettime
------ ----------- ----------- --------- --------- ----------------
100.00    0.012623                 82421      8119 total


Patch (based on kvm-userspace git) attached.



Cheers,
Anders.

diff --git a/qemu/vl.c b/qemu/vl.c
index 80ceb2f..2cd580d 100644
--- a/qemu/vl.c
+++ b/qemu/vl.c
@@ -1063,7 +1063,6 @@ static void qemu_run_timers(QEMUTimer **ptimer_head, 
int64_t current_time)
         /* run the callback (the timer list can be modified) */
         ts->cb(ts->opaque);
     }
-    qemu_rearm_alarm_timer(alarm_timer);
 }
 
 int64_t qemu_get_clock(QEMUClock *clock)
@@ -7216,6 +7215,8 @@ void main_loop_wait(int timeout)
     qemu_run_timers(&active_timers[QEMU_TIMER_REALTIME],
                     qemu_get_clock(rt_clock));
 
+    qemu_rearm_alarm_timer(alarm_timer);
+
     /* Check bottom-halves last in case any of the earlier events triggered
        them.  */
     qemu_bh_poll();