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Fri, 24 Jul 2026 12:35:35 -0700 (PDT) Date: Fri, 24 Jul 2026 15:35:35 -0400 From: Johannes Weiner To: Yosry Ahmed Cc: Nhat Pham , Hao Jia , akpm@linux-foundation.org, tj@kernel.org, shakeel.butt@linux.dev, mhocko@kernel.org, mkoutny@suse.com, chengming.zhou@linux.dev, muchun.song@linux.dev, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Hao Jia Subject: Re: [PATCH v2 2/2] mm/zswap: Support batch writeback in shrink_memcg() Message-ID: References: <20260717085151.22822-1-jiahao.kernel@gmail.com> <20260717085151.22822-3-jiahao.kernel@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 47E411C0002 X-Stat-Signature: 6wg737nq91hmdfsw68hmg9nj43y4iyip X-HE-Tag: 1784921737-912697 X-HE-Meta: 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 VVea5nGg 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Jul 24, 2026 at 11:39:41AM -0700, Yosry Ahmed wrote: > On Fri, Jul 24, 2026 at 11:37 AM Nhat Pham wrote: > > > > On Fri, Jul 24, 2026 at 10:57 AM Yosry Ahmed wrote: > > > > > > The numbers generally look good, the store rejection rate is > > > decreasing and we are naturally doing more writeback which makes > > > sense. The store rejection rate actually increases when the batch size > > > increases to 64, maybe aggressive writeback is more likely to fail and > > > cause a store rejection, so that also makes sense. > > > > > > The only part I can't immediately reason about is pswpin. How are we > > > reading from physical swap more than we ever wrote to it? > > > > The same page can be swapped out once, and swap in multiple times :) > > > > Unlike zswap, physical swap in is not necessarily exclusive. You load > > the page in memory, but if the swapfile is not full, etc. etc. you > > don't invalidate the copy of the data on the swapfile. At reclaim > > time, we notice the page is not dirty, so we just skip (z)swapout. > > Oh right, I forgot about that, thanks. > > I still don't fully understand why pswpin is significantly increased > with batching here. I would hope that with less LRU inversion we end > up with less disk swapin. Probably the test access patterns are just > too random compared to real workloads? Maybe more time in zswap for those tail entries left behind with the smaller batch? That's a 32 entry window that could zswpin with the smaller batch but pswpin with the larger one.