Re: [PATCH V6 4/5] blk-mq-sched: improve dispatching from sw queue

[Ming Lei <ming.lei@redhat.com>]

On Tue, Oct 10, 2017 at 11:23:45AM -0700, Omar Sandoval wrote:
> On Mon, Oct 09, 2017 at 07:24:23PM +0800, Ming Lei wrote:
> > SCSI devices use host-wide tagset, and the shared driver tag space is
> > often quite big. Meantime there is also queue depth for each lun(
> > .cmd_per_lun), which is often small, for example, on both lpfc and
> > qla2xxx, .cmd_per_lun is just 3.
> > 
> > So lots of requests may stay in sw queue, and we always flush all
> > belonging to same hw queue and dispatch them all to driver, unfortunately
> > it is easy to cause queue busy because of the small .cmd_per_lun.
> > Once these requests are flushed out, they have to stay in hctx->dispatch,
> > and no bio merge can participate into these requests, and sequential IO
> > performance is hurt a lot.
> > 
> > This patch introduces blk_mq_dequeue_from_ctx for dequeuing request from
> > sw queue so that we can dispatch them in scheduler's way, then we can
> > avoid to dequeue too many requests from sw queue when ->dispatch isn't
> > flushed completely.
> > 
> > This patch improves dispatching from sw queue when there is per-request-queue
> > queue depth by taking request one by one from sw queue, just like the way
> > of IO scheduler.
> 
> This still didn't address Jens' concern about using q->queue_depth as
> the heuristic for whether to do the full sw queue flush or one-by-one
> dispatch. The EWMA approach is a bit too complex for now, can you please
> try the heuristic of whether the driver ever returned BLK_STS_RESOURCE?

That can be done easily, but I am not sure if it is good.

For example, inside queue rq path of NVMe, kmalloc(GFP_ATOMIC) is
often used, if kmalloc() returns NULL just once, BLK_STS_RESOURCE
will be returned to blk-mq, then blk-mq will never do full sw
queue flush even when kmalloc() always succeed from that time
on.

Even EWMA approach isn't good on SCSI-MQ too, because
some SCSI's .cmd_per_lun is very small, such as 3 on
lpfc and qla2xxx, and one full flush will trigger
BLK_STS_RESOURCE easily.

So I suggest to use the way of q->queue_depth first, since we
don't get performance degrade report on other devices(!q->queue_depth)
with blk-mq. We can improve this way in the future if we
have better approach.

What do you think about it?


-- 
Ming