linux-kernel - Re: volanoMark regression with kernel 2.6.26-rc1

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Message-ID: <20080509155240.GH3612@linux.vnet.ibm.com>
Date:	Fri, 9 May 2008 21:22:40 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: volanoMark regression with kernel 2.6.26-rc1

On Thu, May 08, 2008 at 11:41:25AM +0530, Srivatsa Vaddagiri wrote:
> The other combination that I am interested to know is when:
> 
> CONFIG_FAIR_GROUP_SCHED=y and CONFIG_CGROUP_SCHED=y
> 
> [i.e cgroup based scheduling rather than uid based scheduling. Former
> should result in only one group at bootup]
> 
> I will also try to get some numbers with this combination.

I ran with that combination and here are some results:

2.6.25 (with CONFIG_USER_SCHED) 

	Volanomark perf = 20436.6 (Avg of 10 runs)

2.6.26-rc1 + patches in Ingo's tree [1] as of Fri morning IST (abt 8 hrs
before) (with CONFIG_CGROUP_SCHED)
	
	Volanomark perf = 21529.6

i.e CGROUP based grouping in 2.6.26-rc1 gives same (if not somewhat
better) results as UID-based scheduling in 2.6.25.

Yamin,
	Could you validate this as well? i.e just turn on cgroup-based
grouping (CONFIG_CGROUP_SCHED) and check the resulting performance with 2625
you already have (using CONFIG_USER_SCHED).


A) In 2.6.25, with UID based scheduling,
	CPU load = summation of task load

B) In 2.6.26-rc1, with UID based scheduling,
	CPU load = summation of group weights

C) In 2.6.26-rc1, with CGROUP based scheduling,
	CPU load = summation of task weights


This change in definition of cpu load is affecting load balance routines
(find_busiest_group et al). As a result, threads of volanomark benchmark
aren't quickly spread across the cpus, resulting in slower performance.

In case of B), cpu load can be low numbers (100 or 200), while in A or
C, cpu load are large numbers. I think find_busiest_group() and related 
routines need to be "educated" to deal with such low numbers ..


-- 
Regards,
vatsa
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