From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755450AbcKCNln (ORCPT ); Thu, 3 Nov 2016 09:41:43 -0400 Received: from mx0a-00082601.pphosted.com ([67.231.145.42]:56940 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750756AbcKCNll (ORCPT ); Thu, 3 Nov 2016 09:41:41 -0400 Subject: Re: [PATCH 3/4] blk-mq: implement hybrid poll mode for sync O_DIRECT To: Ming Lei References: <1478034325-28232-1-git-send-email-axboe@fb.com> <1478034325-28232-4-git-send-email-axboe@fb.com> CC: Jens Axboe , Linux Kernel Mailing List , linux-block , Christoph Hellwig From: Jens Axboe Message-ID: <5762c14d-eb4e-b601-a15f-71ef5fbfec3d@fb.com> Date: Thu, 3 Nov 2016 07:41:28 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit 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X-Mailing-List: linux-kernel@vger.kernel.org On 11/03/2016 06:27 AM, Ming Lei wrote: > On Wed, Nov 2, 2016 at 5:05 AM, Jens Axboe wrote: >> This patch enables a hybrid polling mode. Instead of polling after IO >> submission, we can induce an artificial delay, and then poll after that. >> For example, if the IO is presumed to complete in 8 usecs from now, we >> can sleep for 4 usecs, wake up, and then do our polling. This still puts > > I guess in reality it isn't easy to figure a perfect poll time: > > - for one driver, different CPU and different drive/disk may cause different > completion time > > - for requests with different size, the completion time can be different too > > Is there one way to figure out the poll time automatically? Yes, it's not easy to figure out the perfect time, the point is to try and make a guess that's a bit better than the current "let's poll the whole time". I suspect that for most real world cases, you are going to be polling for smallish IO of roughly the same size. Hence the stats should be useful. But we could extend the tracking a bit and make it smarter. -- Jens Axboe