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

[Chengming Zhou]

On 2024/1/18 15:19, Chris Li wrote:
> On Wed, Jan 17, 2024 at 11:02 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>>
>> 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@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.
> 
> 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?

```
#!/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
echo 60 > /proc/sys/vm/swappiness

echo zsmalloc > /sys/module/zswap/parameters/zpool
echo lz4 > /sys/module/zswap/parameters/compressor
echo 1 > /sys/module/zswap/parameters/shrinker_enabled
echo 1 > /sys/module/zswap/parameters/enabled

if ! [ -d $tmpdir ]; then
	mkdir -p $tmpdir
	mount -t tmpfs -o size=100% nodev $tmpdir
fi

mkdir -p $cgroup
echo $memory_max > $cgroup/memory.max
echo $$ > $cgroup/cgroup.procs

rm -rf $workdir
mkdir -p $workdir
cd $workdir

tar xvf $linux_src
cd linux-6.6
make -j$NR_TASK clean
make defconfig
time make -j$NR_TASK
```