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Message-ID: <5f355efb-1091-89b5-546f-8dbbc984f65b@gmail.com> Date: Mon, 24 Feb 2020 12:10:10 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <20200224071211.bar3aqgo76sznqd5@alap3.anarazel.de> Content-Type: text/plain; charset=UTF-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 24/02/2020 10:12, Andres Freund wrote: > Hi, > > On 2020-02-23 20:52:26 -0700, Jens Axboe wrote: >> The fast case is not being deferred, that's by far the common (and hot) >> case, which means io_issue() is called with sqe != NULL. My worry is >> that by moving it into a prep helper, the compiler isn't smart enough to >> not make that basically two switches. > > I'm not sure that benefit of a single switch isn't offset by the lower > code density due to the additional per-opcode branches. Not inlining > the prepare function results in: > The first looks good, I like the change. Do you have performance numbers? e.g. tools/io_uring/io_uring-bench (do_nop=1, with high DEPTH e.g. 100) would be good enough to estimate relative overhead. I don't expect any difference, TBH. > There's still some unnecessary branching on force_nonblocking. The > second patch just separates the cases needing force_nonblocking > out. Probably not quite the right structure. > It's trickier there. It can get into io_prep_issue_sqe_nonblock() -> io_req_prep() with sqe=NULL. With a glance look, it should crash. The culprit is __io_queue_sqe() with linked requests. Also, io_issue_sqe_nonblock() would look better than io_prep_issue_sqe_nonblock(). BTW, did you tried to run regression tests? It's under liburing repository. > > Not quite sure what the policy is with attaching POC patches? Also send > as separate emails? I'd prefer it inlined (i.e. as text, not attachment), so it can be inline-commented. -- Pavel Begunkov