linux-kernel - Re: [PATCH 5/5] mm: x86: Revisit tlb_flushall_shift tuning for page flushes except on IvyBridge

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Date:	Thu, 09 Jan 2014 15:00:51 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>, Alex Shi <alex.shi@...aro.org>,
	Ingo Molnar <mingo@...nel.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.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>
Subject: Re: [PATCH 5/5] mm: x86: Revisit tlb_flushall_shift tuning for page
 flushes except on IvyBridge

On 01/09/2014 09:34 AM, Mel Gorman wrote:
> There was a large ebizzy performance regression that was bisected to commit
> 611ae8e3 (x86/tlb: enable tlb flush range support for x86). The problem
> was related to the tlb_flushall_shift tuning for IvyBridge which was
> altered. The problem is that it is not clear if the tuning values for each
> CPU family is correct as the methodology used to tune the values is unclear.
>
> This patch uses a conservative tlb_flushall_shift value for all CPU families
> except IvyBridge so the decision can be revisited if any regression is found
> as a result of this change. IvyBridge is an exception as testing with one
> methodology determined that the value of 2 is acceptable. Details are in the
> changelog for the patch "x86: mm: Change tlb_flushall_shift for IvyBridge".
>
> One important aspect of this to watch out for is Xen. The original commit
> log mentioned large performance gains on Xen. It's possible Xen is more
> sensitive to this value if it flushes small ranges of pages more frequently
> than workloads on bare metal typically do.
>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Reviewed-by: Rik van Riel <riel@...hat.com>

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