From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753555AbbFQIYU (ORCPT ); Wed, 17 Jun 2015 04:24:20 -0400 Received: from mail-wi0-f169.google.com ([209.85.212.169]:37431 "EHLO mail-wi0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751729AbbFQIYO (ORCPT ); Wed, 17 Jun 2015 04:24:14 -0400 MIME-Version: 1.0 In-Reply-To: <5580E3A0.6050504@unitedstack.com> References: <557EB47F.6090708@unitedstack.com> <557ED1D4.20605@unitedstack.com> <557F97CB.6070608@unitedstack.com> <55800F24.6060100@unitedstack.com> <55802F46.7020804@unitedstack.com> <5580E3A0.6050504@unitedstack.com> Date: Wed, 17 Jun 2015 11:24:12 +0300 Message-ID: Subject: Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object From: Ilya Dryomov To: juncheng bai Cc: idryomov@redhat.com, Alex Elder , Josh Durgin , Guangliang Zhao , jeff@garzik.org, yehuda@hq.newdream.net, Sage Weil , elder@inktank.com, "linux-kernel@vger.kernel.org" , Ceph Development Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 17, 2015 at 6:04 AM, juncheng bai wrote: > Hi. > Yeah, you are right, use the default max_segments, the request size can > be the object size, because the bi_phys_segments of bio could be recount, > there's just a possibility. > > I want to fully understand the bi_phys_segments, hope you can give me some > information, thanks. > > The test information as shown below: > The systemtap script: > global greq=0; > probe kernel.function("bio_attempt_back_merge") > { > greq=pointer_arg(2); > } > > probe kernel.function("bio_attempt_back_merge").return > { > printf("after req addr:%p req segments:%d req offset:%lu req > length:%lu\n", > greq, > @cast(greq, "request")->nr_phys_segments, > @cast(greq, "request")->__sector * 512, > @cast(greq, "request")->__data_len); > } > > probe kernel.function("blk_mq_start_request") > { > printf("req addr:%p nr_phys_segments:%d, offset:%lu len:%lu\n", > pointer_arg(1), > @cast(pointer_arg(1), "request")->nr_phys_segments, > @cast(pointer_arg(1), "request")->__sector * 512, > @cast(pointer_arg(1), "request")->__data_len); > } > > Test command: > dd if=/dev/zero of=/dev/rbd0 bs=4M count=2 oflag=direct seek=100 > > Cast one: > blk_queue_max_segments(q, 256); > > The output of stap: > after req addr:0xffff880ff60a08c0 req segments:73 req offset:419430400 req > length:2097152 > after req addr:0xffff880ff60a08c0 req segments:73 req offset:419430400 req > length:2097152 > after req addr:0xffff880ff60a0a80 req segments:186 req offset:421527552 req > length:1048576 > req addr:0xffff880ff60a08c0 nr_phys_segments:73, offset:419430400 > len:2097152 > req addr:0xffff880ff60a0a80 nr_phys_segments:186, offset:421527552 > len:1048576 > req addr:0xffff880ff60a0c40 nr_phys_segments:232, offset:422576128 > len:1048576 > > after req addr:0xffff880ff60a0c40 req segments:73 req offset:423624704 req > length:2097152 > after req addr:0xffff880ff60a0c40 req segments:73 req offset:423624704 req > length:2097152 > after req addr:0xffff880ff60a0e00 req segments:186 req offset:425721856 req > length:1048576 > req addr:0xffff880ff60a0c40 nr_phys_segments:73, offset:423624704 > len:2097152 > req addr:0xffff880ff60a0e00 nr_phys_segments:186, offset:425721856 > len:1048576 > req addr:0xffff880ff60a0fc0 nr_phys_segments:232, offset:426770432 > len:1048576 > > Case two: > blk_queue_max_segments(q, segment_size / PAGE_SIZE); > > The output of stap: > after req addr:0xffff88101c9a0000 req segments:478 req offset:419430400 req > length:4194304 > req addr:0xffff88101c9a0000 nr_phys_segments:478, offset:419430400 > len:4194304 > > after req addr:0xffff88101c9a0000 req segments:478 req offset:423624704 req > length:4194304 > req addr:0xffff88101c9a0000 nr_phys_segments:478, offset:423624704 > len:4194304 > > 1.Based on the setting of max_sectors and max_segments, decides the > size of a request. > 2.We have already set max_sectors to an object's size, so we should try > to ensure that a request to the size as possible as merge bio. Yeah, I also tried to explain this in the commit description [1]. Initially I had BIO_MAX_PAGES in there, and realistically I still think it's enough for most cases, but discussion with you made me consider readv/writev case and so I changed it in my patch to max_hw_sectors (i.e. segment_size / SECTOR_SIZE) - this ensures that max_segments will never be a limiting factor even in theory. [1] https://github.com/ceph/ceph-client/commit/2d8006795564fbc0fa68d75758f605fe9f7a108e Thanks, Ilya