linux-kernel - Re: [PATCH v1] sched: fix nohz idle load balancer issues

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Message-ID: <20110928141701.GF8593@linux.vnet.ibm.com>
Date:	Wed, 28 Sep 2011 19:47:01 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Venki Pallipadi <venki@...gle.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] sched: fix nohz idle load balancer issues

* Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com> [2011-09-28 10:04:27]:

> > Especially with higher number of logical CPUs. Likely cleaner way is to clear
> > first_pick, second_pick on idle instead of clearing on tickless.
> 
> I think I tried that (cleared first/second_pick_cpu in
> nohz_kick_needed() upon idle) but didn't get the best results. Let me
> try that again and post idle time numbers.

So made some changes so that first/second_pick_cpu identifiers are
cleared when corresponding cpus become idle. They are cleared in
pick_next_task_idle() ..

With that change, I don't see additional benefit of having a third busy
cpu kick ilb_cpu when it has > 1 task.


Idle time -> 			Avg.	Std.Dev   Min   Max
W/o third busy cpu kicking	4.57%   0.9	  3%    8%   
With third busy cpu kicking	4.78%   1.1%      3%    8%

So will go with your suggestion in the next version!

- vatsa
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