Re: [RFC v2 3/3] mm: make swapin readahead to improve thp collapse rate

[Rik van Riel]

On 06/24/2015 08:33 AM, Vlastimil Babka wrote:
> On 06/22/2015 03:37 AM, Rik van Riel wrote:
>> On 06/21/2015 02:11 PM, Kirill A. Shutemov wrote:
>>> On Sat, Jun 20, 2015 at 02:28:06PM +0300, Ebru Akagunduz wrote:
>>>> +	__collapse_huge_page_swapin(mm, vma, address, pmd, pte);
>>>> +
>>>
>>> And now the pages we swapped in are not isolated, right?
>>> What prevents them from being swapped out again or whatever?
>>
>> Nothing, but __collapse_huge_page_isolate is run with the
>> appropriate locks to ensure that once we actually collapse
>> the THP, things are present.
>>
>> The way do_swap_page is called, khugepaged does not even
>> wait for pages to be brought in from swap. It just maps
>> in pages that are in the swap cache, and which can be
>> immediately locked (without waiting).
>>
>> It will also start IO on pages that are not in memory
>> yet, and will hopefully get those next round.
> 
> Hm so what if the process is slightly larger than available memory and really
> doesn't touch the swapped out pages that much? Won't that just be thrashing and
> next round you find them swapped out again?

Yes, it might.

However, all the policy smarts are in patch 2/3, not in
patch 3/3 (which has the mechanism).

I suspect the code could use some more smarts, but I am
not quite sure what they should be...

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