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

[Huang, Ying]

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.

--
Best Regards,
Huang, Ying