Re: [PATCH 2/5] zsmalloc: Consolidate zs_pool's migrate_lock and size_class's locks

[Minchan Kim]

On Mon, Nov 07, 2022 at 01:31:14PM -0800, Nhat Pham wrote:
> We have benchmarked the lock consolidation to see the performance effect of
> this change on zram. First, we ran a synthetic FS workload on a server machine
> with 36 cores (same machine for all runs), using this benchmark script:
> 
> https://github.com/josefbacik/fs_mark
> 
> using 32 threads, and cranking the pressure up to > 80% FS usage.
> 
> Here is the result (unit is file/second):
> 
> With lock consolidation (btrfs):
> Average: 13520.2, Median: 13531.0, Stddev: 137.5961482019028
> 
> Without lock consolidation (btrfs):
> Average: 13487.2, Median: 13575.0, Stddev: 309.08283679298665
> 
> With lock consolidation (ext4):
> Average: 16824.4, Median: 16839.0, Stddev: 89.97388510006668
> 
> Without lock consolidation (ext4)
> Average: 16958.0, Median: 16986.0, Stddev: 194.7370021336469
> 
> As you can see, we observe a 0.3% regression for btrfs, and a 0.9% regression
> for ext4. This is a small, barely measurable difference in my opinion.
> 
> For a more realistic scenario, we also tries building the kernel on zram.
> Here is the time it takes (in seconds):
> 
> With lock consolidation (btrfs):
> real
> Average: 319.6, Median: 320.0, Stddev: 0.8944271909999159
> user
> Average: 6894.2, Median: 6895.0, Stddev: 25.528415540334656
> sys
> Average: 521.4, Median: 522.0, Stddev: 1.51657508881031
> 
> Without lock consolidation (btrfs):
> real
> Average: 319.8, Median: 320.0, Stddev: 0.8366600265340756
> user
> Average: 6896.6, Median: 6899.0, Stddev: 16.04057355583023
> sys
> Average: 520.6, Median: 521.0, Stddev: 1.140175425099138
> 
> With lock consolidation (ext4):
> real
> Average: 320.0, Median: 319.0, Stddev: 1.4142135623730951
> user
> Average: 6896.8, Median: 6878.0, Stddev: 28.621670111997307
> sys
> Average: 521.2, Median: 521.0, Stddev: 1.7888543819998317
> 
> Without lock consolidation (ext4)
> real
> Average: 319.6, Median: 319.0, Stddev: 0.8944271909999159
> user
> Average: 6886.2, Median: 6887.0, Stddev: 16.93221781102523
> sys
> Average: 520.4, Median: 520.0, Stddev: 1.140175425099138
> 
> The difference is entirely within the noise of a typical run on zram. This
> hardly justifies the complexity of maintaining both the pool lock and the class
> lock. In fact, for writeback, we would need to introduce yet another lock to

I am glad to make the zsmalloc lock scheme simpler without meaning
regression since it introduced a lot mess. Please include the test
result in description.

Thanks for the testing, Nhat.