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

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2024/1/19 18:26, Chris Li wrote:
> On Thu, Jan 18, 2024 at 10:19 PM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
>>
>> On 2024/1/19 12:59, Chris Li wrote:
>>> On Wed, Jan 17, 2024 at 11:35 PM Chengming Zhou
>>> <zhouchengming@bytedance.com> wrote:
>>>
>>>>>>>                     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.
>>>>>
>>>>> Thanks for the quick test. Interesting to see the sys usage drop for
>>>>> the xarray case even with the spin lock.
>>>>> Not sure if the 13 second saving is statistically significant or not.
>>>>>
>>>>> We might need to have both xarray and split trees for the zswap. It is
>>>>> likely removing the spin lock wouldn't be able to make up the 35%
>>>>> difference. That is just my guess. There is only one way to find out.
>>>>
>>>> Yes, I totally agree with this! IMHO, concurrent zswap_store paths still
>>>> have to contend for the xarray spinlock even though we would have converted
>>>> the rb-tree to the xarray structure at last. So I think we should have both.
>>>>
>>>>>
>>>>> BTW, do you have a script I can run to replicate your results?
>>>
>>> Hi Chengming,
>>>
>>> Thanks for your script.
>>>
>>>>
>>>> ```
>>>> #!/bin/bash
>>>>
>>>> testname="build-kernel-tmpfs"
>>>> cgroup="/sys/fs/cgroup/$testname"
>>>>
>>>> tmpdir="/tmp/vm-scalability-tmp"
>>>> workdir="$tmpdir/$testname"
>>>>
>>>> memory_max="$((2 * 1024 * 1024 * 1024))"
>>>>
>>>> linux_src="/root/zcm/linux-6.6.tar.xz"
>>>> NR_TASK=32
>>>>
>>>> swapon ~/zcm/swapfile
>>>
>>> How big is your swapfile here?
>>
>> The swapfile is big enough here, I use a 50GB swapfile.
> 
> Thanks,
> 
>>
>>>
>>> It seems you have only one swapfile there. That can explain the contention.
>>> Have you tried multiple swapfiles for the same test?
>>> That should reduce the contention without using your patch.
>> Do you mean to have many 64MB swapfiles to swapon at the same time?
> 
> 64MB is too small. There are limits to MAX_SWAPFILES. It is less than
> (32 - n) swap files.
> If you want to use 50G swap space, you can have MAX_SWAPFILES, each
> swapfile 50GB / MAX_SWAPFILES.

Right.

> 
>> Maybe it's feasible to test,
> 
> Of course it is testable, I am curious to see the test results.
> 
>> I'm not sure how swapout will choose.
> 
> It will rotate through the same priority swap files first.
> swapfile.c: get_swap_pages().
> 
>> But in our usecase, we normally have only one swapfile.
> 
> Is there a good reason why you can't use more than one swapfile?

I think no, but it seems an unneeded change/burden to our admin.
So I just tested and optimized for the normal case.

> One swapfile will not take the full advantage of the existing code.
> Even if you split the zswap trees within a swapfile. With only one
> swapfile, you will still be having lock contention on "(struct
> swap_info_struct).lock".
> It is one lock per swapfile.
> Using more than one swap file should get you better results.

IIUC, we already have the per-cpu swap entry cache to not contend for
this lock? And I don't see much hot of this lock in the testing.

Thanks.