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Received: from willy by casper.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1wD0c5-0000000HW1d-0xZ5; Wed, 15 Apr 2026 13:49:05 +0000 Date: Wed, 15 Apr 2026 14:49:04 +0100 From: Matthew Wilcox To: Mingyu He Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, hannes@cmpxchg.org, clm@meta.com, linux-kernel@vger.kernel.org, kirill@shutemov.name, bfoster@redhat.com, Jens Axboe , littleswimmingwhale@gmail.com Subject: Re: [ISSUE] Read performance regression when using RWF_DONTCACHE form 8026e49 "mm/filemap: add read support for RWF_DONTCACHE" Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2DB56A0004 X-Stat-Signature: zwbrdpg5nsfpkem3scpdw49zootm9zjf X-HE-Tag: 1776260947-65043 X-HE-Meta: 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 CAzEcz6a maEjb9a/UnyZOEE/X5t/cTp7vkifRF1qYqBSSuOuaMHaUq7H1kFnsakc9Ji1bdi7EwKIUG7QbgHylmWp2lWZwqUZ5arYau/pugauLui0PLXgk6DG5cGMLRGyww1PrvvMMKcejkNfQSlCLKX68U8pTBIQU42aevcN5Zx0bi9ZHohhRCMiaHsdXumZ6IalqUa59F5mVEMhJDGFEQpysHPw1TNhyN08gjFzUy/6ri+ry+BlX8lODYwu18EUpgAdRZbAFxTUPpVcnXHmj6aANHoEos8gTZWByaqr7VZLDZK01OqIiTfA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Apr 15, 2026 at 03:28:27PM +0800, Mingyu He wrote: > I found this feature quite useful because in many scenarios, files > only need to be read once and then discarded. Keeping them in the page > cache can lead to a drop in read performance during cache reclaim. > Therefore, I conducted functional tests after the official release of > v7.0.0. I found that in normal preadv2 read scenarios (write has not > been tested yet), the read performance actually has a significant > regression. I would like to discuss whether this is expected, or if my > usage or application scenario is incorrect. Your entire premise is wrong. This is not a magic "make I/O go faster" flag. Comparing it to cached I/O is entirely wrong; your workload clearly benefits from readahead and you've asked to not do readahead by specifying RWF_DONTCACHE. Rather you should compare against O_DIRECT reads.