Re: [monit-dev] control: consume less CPU time while waiting for a process to start/stop

[Martin Pala]

Hi Thomas,

thanks for the patch, the process status polling was really too agressive for 
low power devices. I have refactored the wait_start/wait_stop little bit more:

1.) the wait starts with 50ms and then doubles each cycle until 1s is reached 
(50ms -> 100ms -> 200ms -> 400ms -> 800ms -> 1s) and then monit continues 
polling every 1 second. This way it allows to detect fast starting/stopping 
services state quickly and gradually slow down the polling if the service state 
change takes longer.

2.) the wait will now always sleep for the initial period (50ms) - originally 
the wait was called immediately after start/stop program spawn which was too 
fast in most cases

3.) the wait is made resistant to large time changes

The fix will be part of upcoming Monit 5.4 which should be available tomorrow.

Regards,
Martin



On Apr 11, 2012, at 3:39 PM, Thomas Petazzoni wrote:

> control: consume less CPU time while waiting for a process to start/stop
> 
> Monit uses the wait_start() and wait_stop() functions when waiting for
> a given process to start and stop. In those functions, Monit polls the
> state of those processes to find when they have started or stopped,
> during a given timeout (by default EXEC_TIMEOUT, 30 seconds).
> 
> Unfortunately, during this period, Monit polls very aggressively: it
> polls every 5 milliseconds to see if the process has started or
> not. Since every 5 milliseconds Monit scans a quite important number
> of files in /proc, it makes Monit consumes about 60-70% of the CPU
> time on a faily low-end ARMv5 device (200 Mhz). And when the process
> Monit tries to start effectively never starts (for some reason), then
> Monit consumes 60-70% of the CPU time forever.
> 
> This value of 5 milliseconds being way to aggressive, we take a
> different approach:
> 
> * During the first second, we poll every 200ms to see if the process
>   has started or has stopped
> 
> * During the rest of the timeout period, we poll every second
> 
> Signed-off-by: Thomas Petazzoni <address@hidden>
> 
> Index: b/src/control.c
> ===================================================================
> --- a/src/control.c
> +++ b/src/control.c
> @@ -418,11 +418,18 @@
>         int isrunning = FALSE;
>         time_t timeout = time(NULL) + s->start->timeout;
>         int debug = Run.debug;
> +        unsigned waiting_cycles = 0;
>         ASSERT(s);
>         while ((time(NULL) < timeout) && !Run.stopped) {
>                 if ((isrunning = Util_isProcessRunning(s, TRUE)))
>                         break;
> -                Time_usleep(5000);
> +                /* Test if the process exists every 200ms during the
> +                   first second, and then every second */
> +                if (waiting_cycles < 5)
> +                  Time_usleep(200 * 1000);
> +                else
> +                  Time_usleep(1000 * 1000);
> +                waiting_cycles++;
>                 Run.debug = 0; // Turn off debug second time through to avoid 
> flooding the log with pid file does not exist. This poll stuff here _will_ be 
> refactored away
>         }
>         Run.debug = debug;
> @@ -446,11 +453,18 @@
>         int isrunning = TRUE;
>         time_t timeout = time(NULL) + s->stop->timeout;
>         int debug = Run.debug;
> +        unsigned waiting_cycles = 0;
>         ASSERT(s);
>         while ((time(NULL) < timeout) && !Run.stopped) {
>                 if (! (isrunning = Util_isProcessRunning(s, TRUE)))
>                         break;
> -                Time_usleep(5000);
> +                /* Test if the process stopped every 200ms during the
> +                   first second, and then every second */
> +                if (waiting_cycles < 5)
> +                  Time_usleep(200 * 1000);
> +                else
> +                  Time_usleep(1000 * 1000);
> +                waiting_cycles++;
>                 Run.debug = 0; // Turn off debug second time through to avoid 
> flooding the log with pid file does not exist. This poll stuff here _will_ be 
> refactored away
>         }
>         Run.debug = debug;
> 
> 
> -- 
> Thomas Petazzoni, Free Electrons
> Kernel, drivers, real-time and embedded Linux
> development, consulting, training and support.
> http://free-electrons.com
> 
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