linux-kernel - Re: Linus 2.6.23-rc1

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Date:	Wed, 1 Aug 2007 11:21:34 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Kasper Sandberg <lkml@...anurb.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	CK Mailinglist <ck@....kolivas.org>
Subject: Re: Linus 2.6.23-rc1


On Jul 28 2007 12:34, Linus Torvalds wrote:
>On Sat, 28 Jul 2007, Jan Engelhardt wrote:
>> 
>> Time to investigate...

Well it really is different.

Simple test:
- run Unreal Tournament 99 (nice 0, it gets 98%,99% CPU most of the time)
- in a shell, `renice 20 $$; while :; do date; done;`

The shell only produces one or two outputs per second.
This seems different from the old-2.6 behavior, where a nice-20
process seemed to get a bit more share. (Due to interactivity bonus)


Does anyone have a cpu hog test program that spreads its cpu time
over the second rather than doing 300 ms wake and 700 ms sleep cycles
after another?

>Well, one thing that would be worth doing is to simply create a trace of 
>time-slices for both schedulers.
>
>It could easily be some hacky thing that just saves the process name and 
>TSC at each scheduling event in some fairly small fixed-sized per-CPU 
>circular buffer, and have a /sys interface that reads it out, and then you 
>do
>
>	sleep 60 ; cat /sys/cpubuffer > buffer
>
>and play the game for 60 seconds (so that you get a buffer that represents 
>perhaps the last 10 seconds of play).

Send me patches, I run the test.

>It could *literally* just be an effect of the time quanta used, and CFS 
>just deciding that it's not interactive and giving things too long of a 
>CPU slice.
>
>Yes, it's what "/proc/sys/kernel/sched_granularity_ns" is supposed to 
>tweak, but maybe there's some misfeature there, or maybe the default is 
>just bad for games, or whatever.
>
>Ingo: that sysctl_sched_granularity initialization doesn't make sense. You 
>talk about it being in units of nanoseconds, but then you do
>
>	2000000000ULL/HZ
>
>which is nonsensical. That value is "2 seconds" (not 2ms like the comment 
>says) in nanoseconds, but then divided by HZ, so what's the meaning of 
>that HZ thing? Nothing in the scheduler should care about jiffies, why is 
>that related to HZ? All the scheduler clocks are in ns.
>
>			Linus
>

	Jan
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