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

[Chengming Zhou]

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@xxxxxxxxxx> 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.