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

[Thomas Petazzoni]

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