linux-kernel - Re: busted CFS group load balancer?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1226985548.29743.6.camel@lappy.programming.kicks-ass.net>
Date:	Tue, 18 Nov 2008 06:19:08 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Chris Friesen <cfriesen@...tel.com>
Cc:	Ken Chen <kenchen@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: busted CFS group load balancer?

On Mon, 2008-11-17 at 15:19 -0600, Chris Friesen wrote:
> Ken Chen wrote:
> > On Mon, Nov 17, 2008 at 7:37 AM, Chris Friesen wrote:
> >>> It appears that the fair-group load balancer in 2.6.27 does not work
> >>> properly.
> >> There was an issue fixed post 2.6.27 where the load balancer didn't work
> >> properly if there was one task per group per cpu.  You might try
> >> backporting commit 38736f4 and see if that helps.
> > 
> > Tested git commit 38736f4, it doesn't fix the problem I'm seeing.
> > 
> 
> I plugged in the same weights into my test app (groups 1 and 2 instead 
> of ant/bee) and got the results below for a 10-sec run.  The "actual" 
> numbers give the overall average and then the values for each hog 
> separately.  In this case we see that both tasks in group 2 ended up 
> sharing a cpu with one of the tasks from group 1.
> 
>    group      actual(%)      expected(%)  ctx switches   max_latency(ms)
>        1  99.69(99.38/99.99)   99.81       160/262          4/0
>        2   0.31( 0.31/0.31)     0.19       32/33         391/375
> 
> I've only got a 2-way system.  If the results really are that much worse 
> on larger systems, then that's going to cause problems for us as well. 
> I'll see if I can get some time on a bigger machine.

Note that with larger cpu count and/or lower group weight we'll quickly
run into numerical trouble...

I would recommend trying this with the minimum weight in the order of
8-16 times number of cpus on your system.

There is only so much one can do with 10 bit fixed precision math :/


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/