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Sat, 07 Aug 2021 03:31:16 -0700 (PDT) Received: from [192.168.8.197] ([85.255.237.206]) by smtp.gmail.com with ESMTPSA id x15sm15023813wmc.13.2021.08.07.03.31.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 07 Aug 2021 03:31:16 -0700 (PDT) To: Al Viro Cc: Andrew Morton , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Jens Axboe , linux-kernel@vger.kernel.org References: <07bd408d6cad95166b776911823b40044160b434.1628248975.git.asml.silence@gmail.com> From: Pavel Begunkov Subject: Re: [RFC] mm: optimise generic_file_read_iter Message-ID: Date: Sat, 7 Aug 2021 11:30:48 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 051A790417CD Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=gmail.com header.s=20161025 header.b=H2XHvma4; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf29.hostedemail.com: domain of asmlsilence@gmail.com designates 209.85.221.47 as permitted sender) smtp.mailfrom=asmlsilence@gmail.com X-Stat-Signature: qhi95u4eyo484c6fpggk7oi5rejta777 X-HE-Tag: 1628332277-297904 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000126, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 8/6/21 2:48 PM, 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. >=20 > ... and the same question here. >=20 >> NOTE: as a side effect, it reads inode->i_size after ->direct_IO(), an= d >> I'm not sure whether that's valid, so would be great to get feedback >> from someone who knows better. >=20 > Ought to be safe, I think, but again, how much effect have you observed > from the patch? Answering for both patches -- I haven't benchmarked it and don't expect to find anything just from this one, considering variance between runs. I took a loot at the assembly (gcc 11.1), it removes 2 reads to get i_size, write+read that i_size from stack, because it stashed it on the stack. For example, we've squeezed several percents of throughput before on the io_uring side just by cutting sheer number of not too expensive individually instructions. IMHO, it's easier to do when you spotted something by the way, than rediscovering the same during a performance safari. --=20 Pavel Begunkov