linux-kernel - Re: [PATCH 2/4] mm: Send one IPI per CPU to TLB flush all entries after unmapping pages

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Date:	Wed, 10 Jun 2015 10:33:32 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	H Peter Anvin <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] mm: Send one IPI per CPU to TLB flush all entries
 after unmapping pages


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

> Linear mapped reader on a 4-node machine with 64G RAM and 48 CPUs
> 
>                                         4.1.0-rc6          4.1.0-rc6
>                                           vanilla       flushfull-v6
> Ops lru-file-mmap-read-elapsed   162.88 (  0.00%)   120.81 ( 25.83%)
> 
>            4.1.0-rc6   4.1.0-rc6
>              vanillaflushfull-v6r5
> User          568.96      614.68
> System       6085.61     4226.61
> Elapsed       164.24      122.17
> 
> This is showing that the readers completed 25.83% faster with 30% less
> system CPU time. From vmstats, it is known that the vanilla kernel was
> interrupted roughly 900K times per second during the steady phase of the
> test and the patched kernel was interrupts 180K times per second.
> 
> The impact is lower on a single socket machine.
> 
>                                         4.1.0-rc6          4.1.0-rc6
>                                           vanilla       flushfull-v6
> Ops lru-file-mmap-read-elapsed    25.43 (  0.00%)    20.59 ( 19.03%)
> 
>            4.1.0-rc6    4.1.0-rc6
>              vanilla flushfull-v6
> User           59.14        58.99
> System        109.15        77.84
> Elapsed        27.32        22.31
> 
> It's still a noticeable improvement with vmstat showing interrupts went
> from roughly 500K per second to 45K per second.

Btw., I tried to compare your previous (v5) pfn-tracking numbers with these 
full-flushing numbers, and found that the IRQ rate appears to be the same:

> > From vmstats, it is known that the vanilla kernel was interrupted roughly 900K 
> > times per second during the steady phase of the test and the patched kernel 
> > was interrupts 180K times per second.

> > It's still a noticeable improvement with vmstat showing interrupts went from 
> > roughly 500K per second to 45K per second.

... is that because the batching limit in the pfn-tracking case was high enough to 
not be noticeable in the vmstat?

In the full-flushing case (v6 without patch 4) the batching limit is 'infinite', 
we'll batch as long as possible, right?

Or have I managed to get confused somewhere ...

Thanks,

	Ingo
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