Re: [PATCH 0/2] RFC: zswap tree use xarray instead of RB tree

[Yosry Ahmed]

On Wed, Jan 17, 2024 at 10:57 PM Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> wrote:
>
> Hi Yosry and Chris,
>
> On 2024/1/18 14:39, Yosry Ahmed wrote:
> > On Wed, Jan 17, 2024 at 10:01 PM Yosry Ahmed <yosryahmed@googlecom> wrote:
> >>
> >> That's a long CC list for sure :)
> >>
> >> On Wed, Jan 17, 2024 at 7:06 PM Chris Li <chrisl@xxxxxxxxxx> wrote:
> >>>
> >>> The RB tree shows some contribution to the swap fault
> >>> long tail latency due to two factors:
> >>> 1) RB tree requires re-balance from time to time.
> >>> 2) The zswap RB tree has a tree level spin lock protecting
> >>> the tree access.
> >>>
> >>> The swap cache is using xarray. The break down the swap
> >>> cache access does not have the similar long time as zswap
> >>> RB tree.
> >>
> >> I think the comparison to the swap cache may not be valid as the swap
> >> cache has many trees per swapfile, while zswap has a single tree.
> >>
> >>>
> >>> Moving the zswap entry to xarray enable read side
> >>> take read RCU lock only.
> >>
> >> Nice.
> >>
> >>>
> >>> The first patch adds the xarray alongside the RB tree.
> >>> There is some debug check asserting the xarray agrees with
> >>> the RB tree results.
> >>>
> >>> The second patch removes the zwap RB tree.
> >>
> >> The breakdown looks like something that would be a development step,
> >> but for patch submission I think it makes more sense to have a single
> >> patch replacing the rbtree with an xarray.
> >>
> >>>
> >>> I expect to merge the zswap rb tree spin lock with the xarray
> >>> lock in the follow up changes.
> >>
> >> Shouldn't this simply be changing uses of tree->lock to use
> >> xa_{lock/unlock}? We also need to make sure we don't try to lock the
> >> tree when operating on the xarray if the caller is already holding the
> >> lock, but this seems to be straightforward enough to be done as part
> >> of this patch or this series at least.
> >>
> >> Am I missing something?
> >
> > Also, I assume we will only see performance improvements after the
> > tree lock in its current form is removed so that we get loads
> > protected only by RCU. Can we get some performance numbers to see how
> > the latency improves with the xarray under contention (unless
> > Chengming is already planning on testing this for his multi-tree
> > patches).
>
> I just give it a try, the same test of kernel build in tmpfs with zswap
> shrinker enabled, all based on the latest mm/mm-stable branch.
>
>                     mm-stable           zswap-split-tree    zswap-xarray
> real                1m10.442s           1m4.157s            1m9.962s
> user                17m48.232s          17m41.477s          17m45.887s
> sys                 8m13.517s           5m2.226s            7m59.305s
>
> Looks like the contention of concurrency is still there, I haven't
> look into the code yet, will review it later.

I think that's expected with the current version because the tree
spin_lock is still there and we are still doing lookups with a
spinlock.