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

[Jay Lan <jlan@engr.sgi.com>]

Andrew Morton wrote:
>On Fri, 23 Jun 2006 13:14:41 -0400
>Shailabh Nagar <nagar@watson.ibm.com> wrote:
>
>  
>>The results show that differential between tgid on and off
>>starts becoming significant once the exit rate crosses roughly 1000
>>threads/second. Below that exit rate, the difference is negligible.
>>Above it, the difference starts climbing rapidly.
>>
>>So I guess the question is whether this rate of exit is representative
>>enough of real life to warrant making any more changes to the existing
>>patchset, beyond the locking changes in 2. above.
>>
>>>From my limited experience, I think this is too high an exit rate
>>to be worrying about overhead.
>>
>>    
>
>1000/sec isn't terribly high.  CGI servers, shell scripts.
>
>And kernel development ;) A `pushpatch 1500' here does 992 fork/exec/exit
>per second.
>
>  
>>        %ovhd of tgid on over off
>>        (higher is worse)
>>
>>Exit     User     Sys     Elapsed
>>Rate     Time     Time    Time
>>
>>2283      25.76  649.41   -0.14
>>1193     -10.53   88.81   -0.12
>>963      -11.90    3.28   -0.10
>>806       -8.54   -0.84    0.16
>>694       -4.41    2.38    0.03
>>    
>
>Oh wow.  Something's gone quadratic there.
>  
It was due to a loop in fill_tgid() when per-TG stats
data are assembled for netlink:
        do {
                rc = delayacct_add_tsk(stats, tsk);
                if (rc)
                        break;

        } while_each_thread(first, tsk);

and it is executed inside a lock.
Fortunately single threaded appls do not hit this code.

- jay

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