Re: [PATCH 0/8] mm/swap: optimize swap cache search space

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On Sat, Apr 27, 2024 at 7:16 AM Chris Li <chrisl@xxxxxxxxxx> wrote:
> Hi Ying,
> On Tue, Apr 23, 2024 at 7:26 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >
> > Hi, Matthew,
> >
> > Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:
> >
> > > On Mon, Apr 22, 2024 at 03:54:58PM +0800, Huang, Ying wrote:
> > >> Is it possible to add "start_offset" support in xarray, so "index"
> > >> will subtract "start_offset" before looking up / inserting?
> > >
> > > We kind of have that with XA_FLAGS_ZERO_BUSY which is used for
> > > XA_FLAGS_ALLOC1.  But that's just one bit for the entry at 0.  We could
> > > generalise it, but then we'd have to store that somewhere and there's
> > > no obvious good place to store it that wouldn't enlarge struct xarray,
> > > which I'd be reluctant to do.
> > >
> > >> Is it possible to use multiple range locks to protect one xarray to
> > >> improve the lock scalability?  This is why we have multiple "struct
> > >> address_space" for one swap device.  And, we may have same lock
> > >> contention issue for large files too.
> > >
> > > It's something I've considered.  The issue is search marks.  If we delete
> > > an entry, we may have to walk all the way up the xarray clearing bits as
> > > we go and I'd rather not grab a lock at each level.  There's a convenient
> > > 4 byte hole between nr_values and parent where we could put it.
> > >
> > > Oh, another issue is that we use i_pages.xa_lock to synchronise
> > > address_space.nrpages, so I'm not sure that a per-node lock will help.
> >
> > Thanks for looking at this.
> >
> > > But I'm conscious that there are workloads which show contention on
> > > xa_lock as their limiting factor, so I'm open to ideas to improve all
> > > these things.
> >
> > I have no idea so far because my very limited knowledge about xarray.
> For the swap file usage, I have been considering an idea to remove the
> index part of the xarray from swap cache. Swap cache is different from
> file cache in a few aspects.
> For one if we want to have a folio equivalent of "large swap entry".
> Then the natural alignment of those swap offset on does not make
> sense. Ideally we should be able to write the folio to un-aligned swap
> file locations.

Hi Chris,

This sound interesting, I have a few questions though...

Are you suggesting we handle swap on file and swap on device
differently? Swap on file is much less frequently used than swap on
device I think.

And what do you mean "index part of the xarray"? If we need a cache,
xarray still seems one of the best choices to hold the content.

> The other aspect for swap files is that, we already have different
> data structures organized around swap offset, swap_map and
> swap_cgroup. If we group the swap related data structure together. We
> can add a pointer to a union of folio or a shadow swap entry. We can
> use atomic updates on the swap struct member or breakdown the access
> lock by ranges just like swap cluster does.
> I want to discuss those ideas in the upcoming LSF/MM meet up as well.

Looking forward to it!

> Chris

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