On Wed, Jun 17, 2015 at 6:04 AM, juncheng bai <baijunch...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/