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Fri, 06 Aug 2021 10:18:31 -0700 (PDT) Received: from [192.168.1.116] ([198.8.77.61]) by smtp.gmail.com with ESMTPSA id c24sm13239677pgj.11.2021.08.06.10.18.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 06 Aug 2021 10:18:30 -0700 (PDT) Subject: Re: [RFC] mm: optimise generic_file_read_iter To: Al Viro , Pavel Begunkov Cc: Andrew Morton , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <07bd408d6cad95166b776911823b40044160b434.1628248975.git.asml.silence@gmail.com> From: Jens Axboe Message-ID: Date: Fri, 6 Aug 2021 11:18:28 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=kernel-dk.20150623.gappssmtp.com header.s=20150623 header.b=CKiEOTFM; spf=pass (imf02.hostedemail.com: domain of axboe@kernel.dk designates 209.85.216.49 as permitted sender) smtp.mailfrom=axboe@kernel.dk; dmarc=none X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 2DC6370034AA X-Stat-Signature: cbtg4k65oaf84ir9yj8w1tgdaw4baoby X-HE-Tag: 1628270312-256893 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 8/6/21 7:48 AM, Al Viro wrote: > On Fri, Aug 06, 2021 at 12:42:43PM +0100, Pavel Begunkov wrote: >> Unless direct I/O path of generic_file_read_iter() ended up with an >> error or a short read, it doesn't use inode. So, load inode and size >> later, only when they're needed. This cuts two memory reads and also >> imrpoves code generation, e.g. loads from stack. > > ... and the same question here. > >> NOTE: as a side effect, it reads inode->i_size after ->direct_IO(), and >> I'm not sure whether that's valid, so would be great to get feedback >> from someone who knows better. > > Ought to be safe, I think, but again, how much effect have you observed > from the patch? Ran a quick test here, doing polled IO (~3.3M IOPS) and we reduce the overhead of generic_file_read_iter() from 1.5% of the runtime to 1.2%. Noticeable. Will improve once we stop digging into the inode on the io_uring side. Anyway, just one data point, perhaps Pavel has some too. -- Jens Axboe