Re: [PATCH v4 2/5] zsmalloc: Consolidate zs_pool's migrate_lock and size_class's locks

[Minchan Kim]

On Thu, Nov 17, 2022 at 08:38:36AM -0800, Nhat Pham wrote:
> Currently, zsmalloc has a hierarchy of locks, which includes a
> pool-level migrate_lock, and a lock for each size class. We have to
> obtain both locks in the hotpath in most cases anyway, except for
> zs_malloc. This exception will no longer exist when we introduce a LRU
> into the zs_pool for the new writeback functionality - we will need to
> obtain a pool-level lock to synchronize LRU handling even in zs_malloc.
> 
> In preparation for zsmalloc writeback, consolidate these locks into a
> single pool-level lock, which drastically reduces the complexity of
> synchronization in zsmalloc.
> 
> We have also 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
> 
> fs_mark  -d  ../zram1mnt  -s  100000  -n  2500  -t  32  -k
> 
> before and after for btrfs and ext4 on zram (FS usage is 80%).
> 
> 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 prevent data races on the pool's LRU, further
> complicating the lock handling logic. IMHO, it is just better to
> collapse all of these into a single pool-level lock.
> 
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>