Re: [PATCH 0/2] mm/zswap: optimize the scalability of zswap rb-tree

[Yosry Ahmed]

On Wed, Jan 17, 2024 at 1:23 AM Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> wrote:
>
> When testing the zswap performance by using kernel build -j32 in a tmpfs
> directory, I found the scalability of zswap rb-tree is not good, which
> is protected by the only spinlock. That would cause heavy lock contention
> if multiple tasks zswap_store/load concurrently.
>
> So a simple solution is to split the only one zswap rb-tree into multiple
> rb-trees, each corresponds to SWAP_ADDRESS_SPACE_PAGES (64M). This idea is
> from the commit 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB trunks").
>
> Although this method can't solve the spinlock contention completely, it
> can mitigate much of that contention. Below is the results of kernel build
> in tmpfs with zswap shrinker enabled:
>
>      linux-next  zswap-lock-optimize
> real 1m9.181s    1m3.820s
> user 17m44.036s  17m40.100s
> sys  7m37.297s   4m54.622s
>
> So there are clearly improvements. And it's complementary with the ongoing
> zswap xarray conversion by Chris. Anyway, I think we can also merge this
> first, it's complementary IMHO. So I just refresh and resend this for
> further discussion.

The reason why I think we should wait for the xarray patch(es) is
there is a chance we may see less improvements from splitting the tree
if it was an xarray. If we merge this series first, there is no way to
know.

Chris, do you intend to send the xarray patch(es) anytime soon?