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Thu, 17 Nov 2022 14:02:42 -0800 (PST) Date: Thu, 17 Nov 2022 14:02:40 -0800 From: Minchan Kim To: Nhat Pham Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ngupta@vflare.org, senozhatsky@chromium.org, sjenning@redhat.com, ddstreet@ieee.org, vitaly.wool@konsulko.com Subject: Re: [PATCH v4 2/5] zsmalloc: Consolidate zs_pool's migrate_lock and size_class's locks Message-ID: References: <20221117163839.230900-1-nphamcs@gmail.com> <20221117163839.230900-3-nphamcs@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221117163839.230900-3-nphamcs@gmail.com> ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1668722563; a=rsa-sha256; cv=none; b=oO7DF2TJdvby2T0MtL7Azj2dbQms+m4wD8dpEgDAlJhgNY7gvLsvrtKGfYG1tJs8Q1aBR+ qW/F3sk58ebMaTNpjBbY5iMNQ/GQpzIx2W6cKLS6rV10vAror1Zv314PMWFpmskENDhJl0 Fa7+rLCtrUMv86Q1wq/jzKWkeMacC84= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=NSQnSGeF; 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spf=pass (imf26.hostedemail.com: domain of minchan.kim@gmail.com designates 209.85.214.169 as permitted sender) smtp.mailfrom=minchan.kim@gmail.com X-Rspam-User: X-Stat-Signature: nzy3kfsm3ht6f4m9c5y5i7dseyy6cmhg X-Rspamd-Queue-Id: D71B0140005 X-Rspamd-Server: rspam11 X-HE-Tag: 1668722563-276296 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000002, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > Signed-off-by: Nhat Pham Acked-by: Minchan Kim