Re: [PATCH] mm: don't flush TLB when propagate PTE access bit to struct page.

[Rik van Riel <riel@redhat.com>]

On 10/27/2010 04:19 PM, Ying Han wrote:
>
>
> On Wed, Oct 27, 2010 at 11:05 AM, Rik van Riel <riel@redhat.com
> <mailto:riel@redhat.com>> wrote:
>
>     On 10/27/2010 01:21 PM, Ying Han wrote:
>
>         kswapd's use case of hardware PTE accessed bit is to approximate
>         page LRU.  The
>         ActiveLRU demotion to InactiveLRU are not base on accessed bit,
>         while it is only
>         used to promote when a page is on inactive LRU list.  All of the
>         state transitions
>         are triggered by memory pressure and thus has weak relationship
>         with respect to
>         time.  In addition, hardware already transparently flush tlb
>         whenever CPU context
>         switch processes and given limited hardware TLB resource, the
>         time period in
>         which a page is accessed but not yet propagated to struct page
>         is very small
>         in practice. With the nature of approximation, kernel really
>         don't need to flush TLB
>         for changing PTE's access bit.  This commit removes the flush
>         operation from it.
>
>         Signed-off-by: Ying Han<yinghan@google.com
>         <mailto:yinghan@google.com>>
>         Singed-off-by: Ken Chen<kenchen@google.com
>         <mailto:kenchen@google.com>>
>
>
>     The reasoning behind the patch makes sense.
>
>     However, have you measured any improvements in run time with
>     this patch?  The VM is already tweaked to minimize the number
>     of pages that get aged, so it would be interesting to know
>     where you saw issues.
>
>
> Rik, the workload we were running are some MapReduce jobs.

Well, what kind of performance improvement did you measure?

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