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

[Ying Han <yinghan@google.com>]

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On Wed, Oct 27, 2010 at 11:05 AM, Rik van Riel <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>
>> Singed-off-by: Ken Chen<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.

--Ying

>
> --
> All rights reversed
>

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