On 10/13/2017 10:17 AM, Ming Lei wrote: > On Fri, Oct 13, 2017 at 08:44:23AM -0600, Jens Axboe wrote: >> On 10/12/2017 06:19 PM, Ming Lei wrote: >>> On Thu, Oct 12, 2017 at 12:46:24PM -0600, Jens Axboe wrote: >>>> On 10/12/2017 12:37 PM, Ming Lei wrote: >>>>> For SCSI devices, there is often per-request-queue depth, which need >>>>> to be respected before queuing one request. >>>>> >>>>> The current blk-mq always dequeues one request first, then calls .queue_rq() >>>>> to dispatch the request to lld. One obvious issue of this way is that I/O >>>>> merge may not be good, because when the per-request-queue depth can't be >>>>> respected, .queue_rq() has to return BLK_STS_RESOURCE, then this request >>>>> has to staty in hctx->dispatch list, and never got chance to participate >>>>> into I/O merge. >>>>> >>>>> This patch introduces .get_budget and .put_budget callback in blk_mq_ops, >>>>> then we can try to get reserved budget first before dequeuing request. >>>>> Once we can't get budget for queueing I/O, we don't need to dequeue request >>>>> at all, then I/O merge can get improved a lot. >>>> >>>> I can't help but think that it would be cleaner to just be able to >>>> reinsert the request into the scheduler properly, if we fail to >>>> dispatch it. Bart hinted at that earlier as well. >>> >>> Actually when I start to investigate the issue, the 1st thing I tried >>> is to reinsert, but that way is even worse on qla2xxx. >>> >>> Once request is dequeued, the IO merge chance is decreased a lot. >>> With none scheduler, it becomes not possible to merge because >>> we only try to merge over the last 8 requests. With mq-deadline, >>> when one request is reinserted, another request may be dequeued >>> at the same time. >> >> I don't care too much about 'none'. If perfect merging is crucial for >> getting to the performance level you want on the hardware you are using, >> you should not be using 'none'. 'none' will work perfectly fine for NVMe >> etc style devices, where we are not dependent on merging to the same >> extent that we are on other devices. > > We still have some SCSI device, such as qla2xxx, which is 1:1 multi-queue > device, like NVMe, in my test, the big lock of mq-deadline has been > an issue for this kind of device, and none actually is better than > mq-deadline, even though its merge isn't good. Kyber should be able to fill that hole, hopefully. -- Jens Axboe
