Messages in this thread | | | Date | Wed, 17 Jun 2015 17:47:51 +0800 | From | juncheng bai <> | Subject | Re: [PATCH RFC] storage:rbd: make the size of request is equal to the, size of the object |
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On 2015/6/17 16:24, Ilya Dryomov wrote: > On Wed, Jun 17, 2015 at 6:04 AM, juncheng bai > <baijuncheng@unitedstack.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 > Yeah, I agree with you.
Thanks. ---- juncheng bai
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