Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no swap space

[Huang, Ying]

Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:

> On Mon, Nov 27, 2023 at 7:21 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
>>
>> > On Mon, Nov 27, 2023 at 1:32 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
>> >>
>> >> On Mon, Nov 27, 2023 at 12:22:59AM -0800, Chris Li wrote:
>> >> > On Mon, Nov 27, 2023 at 12:14 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >> > > >  I agree with Ying that anonymous pages typically have different page
>> >> > > > access patterns than file pages, so we might want to treat them
>> >> > > > differently to reclaim them effectively.
>> >> > > > One random idea:
>> >> > > > How about we put the anonymous page in a swap cache in a different LRU
>> >> > > > than the rest of the anonymous pages. Then shrinking against those
>> >> > > > pages in the swap cache would be more effective.Instead of having
>> >> > > > [anon, file] LRU, now we have [anon not in swap cache, anon in swap
>> >> > > > cache, file] LRU
>> >> > >
>> >> > > I don't think that it is necessary.  The patch is only for a special use
>> >> > > case.  Where the swap device is used up while some pages are in swap
>> >> > > cache.  The patch will kill performance, but it is used to avoid OOM
>> >> > > only, not to improve performance.  Per my understanding, we will not use
>> >> > > up swap device space in most cases.  This may be true for ZRAM, but will
>> >> > > we keep pages in swap cache for long when we use ZRAM?
>> >> >
>> >> > I ask the question regarding how many pages can be freed by this patch
>> >> > in this email thread as well, but haven't got the answer from the
>> >> > author yet. That is one important aspect to evaluate how valuable is
>> >> > that patch.
>> >>
>> >> Exactly. Since swap cache has different life time with page cache, they
>> >> would be usually dropped when pages are unmapped(unless they are shared
>> >> with others but anon is usually exclusive private) so I wonder how much
>> >> memory we can save.
>> >
>> > I think the point of this patch is not saving memory, but rather
>> > avoiding an OOM condition that will happen if we have no swap space
>> > left, but some pages left in the swap cache. Of course, the OOM
>> > avoidance will come at the cost of extra work in reclaim to swap those
>> > pages out.
>> >
>> > The only case where I think this might be harmful is if there's plenty
>> > of pages to reclaim on the file LRU, and instead we opt to chase down
>> > the few swap cache pages. So perhaps we can add a check to only set
>> > sc->swapcache_only if the number of pages in the swap cache is more
>> > than the number of pages on the file LRU or similar? Just make sure we
>> > don't chase the swapcache pages down if there's plenty to scan on the
>> > file LRU?
>>
>> The swap cache pages can be divided to 3 groups.
>>
>> - group 1: pages have been written out, at the tail of inactive LRU, but
>>   not reclaimed yet.
>>
>> - group 2: pages have been written out, but were failed to be reclaimed
>>   (e.g., were accessed before reclaiming)
>>
>> - group 3: pages have been swapped in, but were kept in swap cache.  The
>>   pages may be in active LRU.
>>
>> The main target of the original patch should be group 1.  And the pages
>> may be cheaper to reclaim than file pages.
>>
>> Group 2 are hard to be reclaimed if swap_count() isn't 0.
>>
>> Group 3 should be reclaimed in theory, but the overhead may be high.
>> And we may need to reclaim the swap entries instead of pages if the pages
>> are hot.  But we can start to reclaim the swap entries before the swap
>> space is run out.
>>
>> So, if we can count group 1, we may use that as indicator to scan anon
>> pages.  And we may add code to reclaim group 3 earlier.
>>
>
> My point was not that reclaiming the pages in the swap cache is more
> expensive that reclaiming the pages in the file LRU. In a lot of
> cases, as you point out, the pages in the swap cache can just be
> dropped, so they may be as cheap or cheaper to reclaim than the pages
> in the file LRU.
>
> My point was that scanning the anon LRU when swap space is exhausted
> to get to the pages in the swap cache may be much more expensive,
> because there may be a lot of pages on the anon LRU that are not in
> the swap cache, and hence are not reclaimable, unlike pages in the
> file LRU, which should mostly be reclaimable.
>
> So what I am saying is that maybe we should not do the effort of
> scanning the anon LRU in the swapcache_only case unless there aren't a
> lot of pages to reclaim on the file LRU (relatively). For example, if
> we have a 100 pages in the swap cache out of 10000 pages in the anon
> LRU, and there are 10000 pages in the file LRU, it's probably not
> worth scanning the anon LRU.

For group 1 pages, they are at the tail of the anon inactive LRU, so the
scan overhead is low too.  For example, if number of group 1 pages is
100, we just need to scan 100 pages to reclaim them.  We can choose to
stop scanning when the number of the non-group-1 pages reached some
threshold.

--
Best Regards,
Huang, Ying