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

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Chris Li <chrisl@xxxxxxxxxx> writes:

> 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.
> 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.

The shadow swap entry may be freed.  So we need to prepare for that.
And, in current design, only swap_map[] is allocated if the swap space
isn't used.  That needs to be considered too.

> We can use atomic updates on the swap struct member or breakdown the
> access lock by ranges just like swap cluster does.

The swap code uses xarray in a simple way.  That gives us opportunity to
optimize.  For example, it makes it easy to use multiple xarray
instances for one swap device.

> I want to discuss those ideas in the upcoming LSF/MM meet up as well.


Best Regards,
Huang, Ying

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