Re: [PATCH 0/7] mm/zswap: optimize the scalability of zswap rb-tree

[Chengming Zhou]

On 2023/12/7 04:08, Nhat Pham wrote:
> On Wed, Dec 6, 2023 at 1:46 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.
> 
> By how much? Do you have any stats to estimate the amount of
> contention and the reduction by this patch?

Actually, I did some test using the linux-next 20231205 yesterday.

Testcase: memory.max = 2G, zswap enabled, make -j32 in tmpfs.

			20231205	+patchset
1. !shrinker_enabled:   156s		126s
2.  shrinker_enabled:   79s		70s

I think your zswap shrinker fix patch can solve !shrinker_enabled case.

So will test again today using the new mm-unstable branch.

> 
> I do think lock contention could be a problem here, and it will be
> even worse with the zswap shrinker enabled (which introduces an
> theoretically unbounded number of concurrent reclaimers hammering on
> the zswap rbtree and its lock). I am generally a bit weary about
> architectural change though, especially if it is just a bandaid. We
> have tried to reduce the lock contention somewhere else (multiple
> zpools), and as predicted it just shifts the contention point
> elsewhere. Maybe we need a deeper architectural re-think.
> 
> Not an outright NACK of course - just food for thought.
> 

Right, I think xarray is good for lockless reading side, and
multiple trees is also complementary, which can reduce the lock
contention on the writing sides too.

>>
>> Another problem when testing the zswap using our default zsmalloc is that
>> zswap_load() and zswap_writeback_entry() have to malloc a temporary memory
>> to support !zpool_can_sleep_mapped().
>>
>> Optimize it by reusing the percpu crypto_acomp_ctx->dstmem, which is also
>> used by zswap_store() and protected by the same percpu crypto_acomp_ctx->mutex.
> 
> It'd be nice to reduce the (temporary) memory allocation on these
> paths, but would this introduce contention on the per-cpu dstmem and
> the mutex that protects it, if there are too many concurrent
> store/load/writeback requests?

I think the mutex holding time is not changed, right? So the contention
on the per-cpu mutex should be the same. We just reuse percpu dstmem more.

Thanks!