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SubjectRe: scsi-mq V2
On 06/25/2014 10:50 PM, Jens Axboe wrote:
> 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.

Ran stress testing from Friday to now, 65h of beating up on it and no
problems observed. 47TB read and 20TB written for a total of 17.7
billion of IOs issued and completed. Latencies look good. I officially
declare this code for bug free.

Bug-free-by: Jens Axboe <>

Now lets get this queued up for inclusion, pretty please.

Jens Axboe

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