by juncheng bai

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

On 2015/6/17 16:24, Ilya Dryomov wrote:
> On Wed, Jun 17, 2015 at 6:04 AM, juncheng bai
> <[email protected]> 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