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 <[email protected]> > --- > 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, so I think we need to resize the hw tags in blk_mq_init_sched() if we're using hardware tags.
