linux-kernel - Re: [PATCH 00/27] Latest numa/core release, v16

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Date:	Mon, 19 Nov 2012 23:00:35 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 00/27] Latest numa/core release, v16

On Mon, Nov 19, 2012 at 11:36:04PM +0100, Ingo Molnar wrote:
> 
> * Mel Gorman <mgorman@...e.de> wrote:
> 
> > Ok.
> > 
> > In response to one of your later questions, I found that I had 
> > in fact disabled THP without properly reporting it. [...]
> 
> Hugepages is a must for most forms of NUMA/HPC.

Requiring huge pages to avoid a regression is a mistake.

> This alone 
> questions the relevance of most of your prior numa/core testing 
> results. I now have to strongly dispute your other conclusions 
> as well.
> 

I'll freely admit that disabling THP for specjbb was a mistake and I should
have caught why at the start. However, the autonumabench figures reported for
the last release had THP enabled as had the kernel build benchmark figures.

> Just a look at 'perf top' output should have told you the story.
> 

I knew THP were not in use and said so in earlier reports. Take this for
example -- https://lkml.org/lkml/2012/11/16/207 . For specjbb, note that
the THP fault alloc figures are close to 0 and due to that I said "THP is
not really a factor for this workload". What I failed to do was identify
why THP was not in use.

> Yet time and time again you readily reported bad 'schednuma' 
> results for a slow 4K memory model that neither we nor other 
> NUMA testers I talked to actually used, without stopping to look 
> why that was so...
> 

Again, I apologise for the THP mistake. The fact remains that the other
implementations did not suffer a performance slowdown due to the same
mistake.

> [ I suspect that if such terabytes-of-data workloads are forced 
>   through such a slow 4K pages model then there's a bug or 
>   mis-tuning in our code that explains the level of additional 
>   slowdown you saw - we'll fix that.
> 
>   But you should know that behavior under the slow 4K model 
>   tells very little about the true scheduling and placement 
>   quality of the patches... ]
> 
> Please report proper THP-enabled numbers before continuing.
> 

Will do. Are THP-disabled benchmark results to be ignored?

-- 
Mel Gorman
SUSE Labs
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