On Thu, May 4, 2017 at 12:46 AM, Omar Sandoval <osandov@xxxxxxxxxxx> wrote: > On Fri, Apr 28, 2017 at 11:15:36PM +0800, Ming Lei wrote: >> When blk-mq I/O scheduler is used, we need two tags for >> submitting one request. One is called scheduler tag for >> allocating request and scheduling I/O, another one is called >> driver tag, which is used for dispatching IO to hardware/driver. >> This way introduces one extra per-queue allocation for both tags >> and request pool, and may not be as efficient as case of none >> scheduler. >> >> Also currently we put a default per-hctx limit on schedulable >> requests, and this limit may be a bottleneck for some devices, >> especialy when these devices have a quite big tag space. >> >> This patch introduces BLK_MQ_F_SCHED_USE_HW_TAG so that we can >> allow to use hardware/driver tags directly for IO scheduling if >> devices's hardware tag space is big enough. Then we can avoid >> the extra resource allocation and make IO submission more >> efficient. >> >> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> >> --- >> block/blk-mq-sched.c | 10 +++++++++- >> block/blk-mq.c | 35 +++++++++++++++++++++++++++++------ >> include/linux/blk-mq.h | 1 + >> 3 files changed, 39 insertions(+), 7 deletions(-) > > One more note on this: if we're using the hardware tags directly, then > we are no longer limited to q->nr_requests requests in-flight. Instead, > we're limited to the hw queue depth. We probably want to maintain the > original behavior, That need further investigation, and generally scheduler should be happy with more requests which can be scheduled. We can make it as one follow-up. > so I think we need to resize the hw tags in blk_mq_init_sched() if we're using hardware tags. That might not be good since hw tags are used by both scheduler and dispatching. Thanks, Ming Lei
