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Date:	Wed, 17 Jun 2015 11:24:12 +0300
From:	Ilya Dryomov <idryomov@...il.com>
To:	juncheng bai <baijuncheng@...tedstack.com>
Cc:	idryomov@...hat.com, Alex Elder <elder@...aro.org>,
	Josh Durgin <josh.durgin@...tank.com>,
	Guangliang Zhao <lucienchao@...il.com>, jeff@...zik.org,
	yehuda@...newdream.net, Sage Weil <sage@...dream.net>,
	elder@...tank.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ceph Development <ceph-devel@...r.kernel.org>
Subject: Re: [PATCH RFC] storage:rbd: make the size of request is equal to
 the, size of the object

On Wed, Jun 17, 2015 at 6:04 AM, juncheng bai
<baijuncheng@...tedstack.com> 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
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