linux-kernel - Re: Linus 2.6.23-rc1

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Date:	Sat, 28 Jul 2007 12:34:57 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
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 Sat, 28 Jul 2007, Jan Engelhardt wrote:
> 
> Time to investigate...

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).

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
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