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 9:39 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
>>
>> > On Mon, Nov 27, 2023 at 8:05 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >>
>> >> 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.
>> >>
>> >
>> > We should still try to reclaim pages in groups 2 & 3 before OOMing
>> > though. Maybe the motivation for this patch is group 1, but I don't
>> > see why we should special case them. Pages in groups 2 & 3 should be
>> > roughly equally cheap to reclaim. They may have higher refault cost,
>> > but IIUC we should still try to reclaim them before OOMing.
>>
>> The scan cost of group 3 may be high, you may need to scan all anonymous
>> pages to identify them.  The reclaim cost of group 2 may be high, it may
>> just cause trashing (shared pages that are accessed by just one
>> process).  So I think that we can allow reclaim group 1 in all cases.
>> Try to reclaim swap entries for group 3 during normal LRU scanning after
>> more than half of swap space of limit is used.  As a last resort before
>> OOM, try to reclaim group 2 and group 3.  Or, limit scan count for group
>> 2 and group 3.
>
> It would be nice if this can be done auto-magically without having to
> keep track of the groups separately.

Some rough idea may be

- trying to scan anon LRU if there are swap cache pages.

- if some number of pages other than group 1 encountered, stop scanning
  anon LRU list.

- the threshold to stopping can be tuned according to whether we are
  going to OOM.

We can try to reclaim swap entries for group 3 when we haven't run out
of swap space yet.

>>
>> BTW, in some situation, OOM is not the worst situation.  For example,
>> trashing may kill interaction latency, while killing the memory hog (may
>> be caused by memory leak) saves system response time.
>
> I agree that in some situations OOMs are better than thrashing, it's
> not an easy problem.

--
Best Regards,
Huang, Ying