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SubjectRe: scsi-mq V2
On 2014-06-25 10:51, Christoph Hellwig wrote:
> This is the second post of the scsi-mq series.
> At this point the code is ready for merging and use by developers and early
> adopters. The core blk-mq code isn't that suitable for slow devices
> yet, mostly due to the lack of an I/O scheduler, but Jens is working on it.
> Similarly there is no dm-multipath support for drivers using blk-mq yet,
> but I'm working on it. It should also be noted that the code doesn't
> actually support multiple hardware queues or fine grained tuning of the
> blk-mq parameters yet. All these could be added fairly easily as soon
> as low-level drivers want to make use of them.
> The amount of chances to the existing code are fairly small, and mostly
> speedups or cleanups that also apply to the old path as well. Because
> of this I also haven't bothered to put it under a config option, just
> like the blk-mq core.
> The usage of blk-mq dramatically decreases CPU usage under all workloads going
> down from 100% CPU usage that the old setup can hit easily to usually less
> than 20% for maxing out storage subsystems with 512byte reads and writes,
> and it allows to easily archive millions of IOPS. Bart and Robert have
> helped with some very detailed measurements that they might be able to send
> in reply to this, although these usually involve significantly reworked low
> level drivers to avoid other bottle necks.
> One major objection to previous iterations of this code was the simple
> replacement of the host_lock with atomic counters for the host and busy
> counters. The host_lock avoidance on it's own already improves performance,
> and with the patch to avoid maintaining the per-target busy counter unless
> needed we now replace a lock round trip on the host_lock with just a single
> atomic increment in the submission path, and a single atomic decrement in
> completion path, which should provide benefits even for the oddest RISC
> architecture. Longer term I'd still love to get rid of these entirely
> and use the counters in blk-mq, but due to the difference in how they
> are maintained this doesn't seem feasible as long as we still need to
> support the legacy request code path.
> Changes from V1:
> - rebased on top of the core-for-3.17 branch, most notable the
> scsi logging changes
> - fixed handling of cmd_list to prevent crashes for some heavy
> workloads
> - fixed incorrect handling of !target->can_queue
> - avoid scheduling a workqueue on I/O completions when no queues
> are congested
> In addition to the patches in this thread there also is a git available at:
> git:// scsi-mq.2

You can add my acked/reviewed-by to the series.

Jens Axboe

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