Date: Fri, 13 Aug 2004 03:02:36 -0700 (PDT) From: Don Lewis <truckman@FreeBSD.org> To: rwatson@FreeBSD.org Cc: freebsd-current@FreeBSD.org Subject: Re: nice handling in ULE (was: Re: SCHEDULE and high load situations) Message-ID: <200408131002.i7DA2afX001590@gw.catspoiler.org> In-Reply-To: <200408122239.i7CMddjc021563@gw.catspoiler.org>
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On 12 Aug, Don Lewis wrote:
> I did some experimentation, and the problem I'm seeing appears to just
> be related to how nice values are handled by ULE. I'm running two
> copies of the following program, one at nice +15, and the other not
> niced:
>
> hairball:~ 102>cat sponge.c
> int
> main(int argc, char **argv)
> {
> while (1)
> ;
> }
>
> The niced process was started second, but it has accumulated more CPU
> time and is getting a larger percentage of the CPU time according to
> top.
>
> last pid: 662; load averages: 2.00, 1.95, 1.45 up 0+00:22:35 15:14:27
> 31 processes: 3 running, 28 sleeping
> CPU states: 45.3% user, 53.1% nice, 1.2% system, 0.4% interrupt, 0.0% idle
> Mem: 22M Active, 19M Inact, 44M Wired, 28K Cache, 28M Buf, 408M Free
> Swap: 1024M Total, 1024M Free
> Seconds to delay:
> PID USERNAME PRI NICE SIZE RES STATE TIME WCPU CPU COMMAND
> 599 dl 139 15 1180K 448K RUN 8:34 53.91% 53.91% sponge
> 598 dl 139 0 1180K 448K RUN 7:22 42.97% 42.97% sponge
> 587 dl 76 0 2288K 1580K RUN 0:03 0.00% 0.00% top
> 462 root 76 0 56656K 46200K select 0:02 0.00% 0.00% Xorg
> 519 gdm 76 0 11252K 8564K select 0:01 0.00% 0.00% gdmlogin
> 579 dl 76 0 6088K 2968K select 0:00 0.00% 0.00% sshd
>
>
>
> I thought it might have something to do with grouping by niceness, which
> would group the un-niced process with a bunch of other processes that
> wake up every now and then for a little bit if CPU time, so I tried the
> experiment again with nice +1 and nice +15. This gave a rather
> interesting result. Top reports the nice +15 process as getting a
> higher %CPU, but the nice +1 process has slowly accumulated a bit more
> total CPU time. The difference in total CPU time was initially seven
> seconds or less.
>
> last pid: 745; load averages: 2.00, 1.99, 1.84 up 0+00:43:30 15:35:22
> 31 processes: 3 running, 28 sleeping
> CPU states: 0.0% user, 99.6% nice, 0.4% system, 0.0% interrupt, 0.0% idle
> Mem: 22M Active, 19M Inact, 44M Wired, 28K Cache, 28M Buf, 408M Free
> Swap: 1024M Total, 1024M Free
> Seconds to delay:
> PID USERNAME PRI NICE SIZE RES STATE TIME WCPU CPU COMMAND
> 675 dl 139 15 1180K 448K RUN 9:48 52.34% 52.34% sponge
> 674 dl 139 1 1180K 448K RUN 10:03 44.53% 44.53% sponge
> 587 dl 76 0 2288K 1580K RUN 0:06 0.00% 0.00% top
> 462 root 76 0 56656K 46200K select 0:03 0.00% 0.00% Xorg
> 519 gdm 76 0 11252K 8564K select 0:02 0.00% 0.00% gdmlogin
> 579 dl 76 0 6088K 2968K select 0:00 0.00% 0.00% sshd
I compiled a kernel with the KTR stuff and ran this last experiment
again. It looks like the two niced processes get the appropriate slice
values assigned by ULE, and they both have the same priority. Where
things seem to be going wrong is that the two processes are being run in
a round robin fashion, alternating execution once every tick or two. The
less-nice process gets prempted multiple times by the more-nice process
before the less-nice process has exhausted its slice.
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