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

[Minchan Kim]

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.

> Regarding running out of swap space. That is a good point, in server
> workload we don't typically run out of swap device space anyway.
> 
> Android uses ZRAM, the story might be different. Adding Minchan here.

Swap is usually almost full in Android since it compacts(i.e., swapout)
background apps aggressively.