linux-kernel - Re: [RFC]x86: clearing access bit don't flush tlb

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Date:	Mon, 07 Jan 2013 10:14:51 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Shaohua Li <shli@...nel.org>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mingo@...hat.com, hpa@...or.com
Subject: Re: [RFC]x86: clearing access bit don't flush tlb

On 01/07/2013 03:12 AM, Shaohua Li wrote:
>
> We use access bit to age a page at page reclaim. When clearing pte access bit,
> we could skip tlb flush for the virtual address. The side effect is if the pte
> is in tlb and pte access bit is unset, when cpu access the page again, cpu will
> not set pte's access bit. So next time page reclaim can reclaim hot pages
> wrongly, but this doesn't corrupt anything. And according to intel manual, tlb
> has less than 1k entries, which coverers < 4M memory. In today's system,
> several giga byte memory is normal. After page reclaim clears pte access bit
> and before cpu access the page again, it's quite unlikely this page's pte is
> still in TLB. Skiping the tlb flush for this case sounds ok to me.

Agreed. In current systems, it can take a minute to write
all of memory to disk, while context switch (natural TLB
flush) times are in the dozens-of-millisecond timeframes.

> And in some workloads, TLB flush overhead is very heavy. In my simple
> multithread app with a lot of swap to several pcie SSD, removing the tlb flush
> gives about 20% ~ 30% swapout speedup.
>
> Signed-off-by: Shaohua Li <shli@...ionio.com>

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


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