Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object

[juncheng bai]

On 2015/6/17 16:24, Ilya Dryomov wrote:
> On Wed, Jun 17, 2015 at 6:04 AM, juncheng bai
> <baijuncheng@xxxxxxxxxxxxxxx> 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
Yeah, I agree with you.

Thanks.
----
juncheng bai
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