Re: [PATCH v2 00/12] mm: page migration enhancement for thp

[Zi Yan]

On 9 Nov 2016, at 18:52, Naoya Horiguchi wrote:

> Hi Anshuman,
>
> On Wed, Nov 09, 2016 at 04:03:04PM +0530, Anshuman Khandual wrote:
>> On 11/08/2016 05:01 AM, Naoya Horiguchi wrote:
>>> Hi everyone,
>>>
>>> I've updated thp migration patches for v4.9-rc2-mmotm-2016-10-27-18-27
>>> with feedbacks for ver.1.
>>>
>>> General description (no change since ver.1)
>>> ===========================================
>>>
>>> This patchset enhances page migration functionality to handle thp migration
>>> for various page migration's callers:
>>>  - mbind(2)
>>>  - move_pages(2)
>>>  - migrate_pages(2)
>>>  - cgroup/cpuset migration
>>>  - memory hotremove
>>>  - soft offline
>>>
>>> The main benefit is that we can avoid unnecessary thp splits, which helps us
>>> avoid performance decrease when your applications handles NUMA optimization on
>>> their own.
>>>
>>> The implementation is similar to that of normal page migration, the key point
>>> is that we modify a pmd to a pmd migration entry in swap-entry like format.
>>
>> Will it be better to have new THP_MIGRATE_SUCCESS and THP_MIGRATE_FAIL
>> VM events to capture how many times the migration worked without first
>> splitting the huge page and how many time it did not work ?
>
> Thank you for the suggestion.
> I think that's helpful, so will try it in next version.
>
>> Also do you
>> have a test case which demonstrates this THP migration and kind of shows
>> its better than the present split and move method ?
>
> I don't have test cases which compare thp migration and split-then-migration
> with some numbers. Maybe measuring/comparing the overhead of migration is
> a good start point, although I think the real benefit of thp migration comes
> from workload "after migration" by avoiding thp split.

Migrating 4KB pages has much lower (~1/3) throughput than 2MB pages.

What I get is that on average it takes 1987.38 us to migrate 512 4KB pages and
                                       658.54  us to migrate 1   2MB page.

I did the test in a two-socket Intel Xeon E5-2640v4 box. I used migrate_pages()
system call to migrate pages. MADV_NOHUGEPAGE and MADV_HUGEPAGE are used to
make 4KB and 2MB pages and each pageâs flags are checked to make sure the page
size is 4KB or 2MB THP.

There is no split page. But the page migration time already tells the story.

â
Best Regards,
Yan Zi

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