linux-kernel - Re: [PATCH 0/4] Fix ebizzy performance regression due to X86 TLB range flush v2

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Date:	Mon, 16 Dec 2013 14:44:49 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alex Shi <alex.shi@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	H Peter Anvin <hpa@...or.com>, Linux-X86 <x86@...nel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 0/4] Fix ebizzy performance regression due to X86 TLB
 range flush v2

* Mel Gorman <mgorman@...e.de> wrote:

> > Whatever we did right in v3.4 we want to do in v3.13 as well - or 
> > at least understand it.
> 
> Also agreed. I started a bisection before answering this mail. It 
> would be cooler and potentially faster to figure it out from direct 
> analysis but bisection is reliable and less guesswork.

Trying to guess can potentially last a _lot_ longer than a generic, 
no-assumptions bisection ...

The symptoms could point to anything: scheduler, locking details, some 
stupid little change in a wakeup sequence somewhere, etc.

It might even be a non-deterministic effect of some timing change 
causing the workload 'just' to avoid a common point of preemption and 
not scheduling as much - and become more unfair and thus certain 
threads lasting longer to finish.

Does the benchmark execute a fixed amount of transactions per thread? 

That might artificially increase the numeric regression: with more 
threads it 'magnifies' any unfairness effects because slower threads 
will become slower, faster threads will become faster, as the thread 
count increases.

[ That in itself is somewhat artificial, because real workloads tend 
  to balance between threads dynamically and don't insist on keeping 
  the fastest threads idle near the end of a run. It does not
  invalidate the complaint about the unfairness itself, obviously. ]

	Ingo
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