There is a defect for blk-mq compare to blk-sq, specifically split io will end up discontinuous if the device is under high io pressure, while split io will still be continuous in sq, this is because: 1) split bio is issued one by one, if one bio can't get tag, it will go to wail. - patch 2 2) each time 8(or wake batch) requests is done, 8 waiters will be woken up. Thus if a thread is woken up, it will unlikey to get multiple tags. - patch 3,4 3) new io can preempt tag even if there are lots of threads waiting for tags. - patch 5 Test environment: x86 vm, nr_requests is set to 64, queue_depth is set to 32 and max_sectors_kb is set to 128. I haven't tested this patchset on physical machine yet, I'll try later if anyone thinks this approch is meaningful. Fio test cmd: [global] filename=/dev/sda ioengine=libaio direct=1 offset_increment=100m [test] rw=randwrite bs=512k numjobs=256 iodepth=2 Result: raito of sequential io(calculated from log by blktrace) original: 21% patched: split io thoroughly and wake up based on required tags. 40% patched and set flag: disable tag preemption. 69% Yu Kuai (6): blk-mq: add a new flag 'BLK_MQ_F_NO_TAG_PREEMPTION' block: refactor to split bio thoroughly blk-mq: record how many tags are needed for splited bio sbitmap: wake up the number of threads based on required tags blk-mq: don't preempt tag expect for split bios sbitmap: force tag preemption if free tags are sufficient block/bio.c | 2 + block/blk-merge.c | 95 ++++++++++++++++++++++++++++----------- block/blk-mq-debugfs.c | 1 + block/blk-mq-tag.c | 39 +++++++++++----- block/blk-mq.c | 14 +++++- block/blk-mq.h | 7 +++ block/blk.h | 3 +- include/linux/blk-mq.h | 7 ++- include/linux/blk_types.h | 6 +++ include/linux/sbitmap.h | 8 ++++ lib/sbitmap.c | 33 +++++++++++++- 11 files changed, 173 insertions(+), 42 deletions(-) -- 2.31.1
