Re: [Patch][RFC] Disabling per-tgid stats on task exit in taskstats

[Shailabh Nagar <nagar@watson.ibm.com>]

Andrew Morton wrote:
> 
> You see the problem - if one userspace package wants the tgid-stats and
> another concurrently-running one does now, what do we do?  Just leave it
> enabled and run a bit slower?
> 
> If so, how much slower?  Your changelog says some potential users don't
> need the tgid-stats, but so what?  I assume this patch is a performance
> thing?  If so, has it been quantified?


Here are some results from running a simple program (source below) that does
10 iterations of creating and then destroying 1000 threads. On the side, another utility
kept reading the pid (+tgid if present) stats from exiting tasks.


	Yes	No	Ovhd
user	0.14	0.15	-6%
system	1.61	1.54	+5%
elapsed	2.01	1.94	+3%

Yes = tgid stats printed on exit
No = not printed
Ovhd = (Yes-No)/No * 100

So even in this extreme case where the per-tgid stats are indeed
half of the total data, the overhead is not very significant.

As pointed out earlier, more representative cases are
- single threaded apps (e.g. make -jX) where the current
taskstats interface already optimizes by not sending redundant per-tgid stats, or
- server-type multithreaded apps where the exits are going to be relatively infrequent (due to
reuse of thread pools) so the extra per-tgid output is not going to have much impact.

I'd suggest we drop the idea of including this patch until we have data showing that
the overhead is an issue.

--Shailabh



#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <pthread.h>

int n;

void *slow_exit(void *arg)
{
	int i = (int) arg;
	usleep((n-i)*2);
}

int main(int argc, char *argv[])
{
	int i,rc, rep;
	pthread_t *ppthread;
	
	n = 5 ;
	if (argc > 1)
		n = atoi(argv[1]);

	rep = 10;
	if (argc > 2)
		rep = atoi(argv[2]);

	ppthread = malloc(n * sizeof(pthread_t));
	if (ppthread == NULL) {
		printf("Memory allocation failure\n");
		exit(-1);
	}

	while (rep) {
		for (i=0; i<n; i++) {
			rc = pthread_create(&ppthread[i], NULL,
					    slow_exit, (void *)i);
			if (rc) {
				printf("Error creating thread %d\n", i);
				exit(-1);
			}
		}
		for (i=0; i<n; i++) {
			rc = pthread_join(ppthread[i], NULL);
			if (rc) {
				printf("Error joining thread %d\n", i);
				exit(-1);
			}
		}
		rep--;
	}
}