[dm-devel] blk-mq request allocation stalls [was: Re: [PATCH v3 0/8] dm: add request-based blk-mq support]

[Jens Axboe]

On 01/07/2015 03:33 AM, Bart Van Assche wrote:
> On 01/06/15 17:15, Jens Axboe wrote:
>> blk-mq request allocation is pretty much as optimized/fast as it can be.
>> The slowdown must be due to one of two reasons:
>>
>> - A bug related to running out of requests, perhaps a missing queue run
>> or something like that.
>> - A smaller number of available requests, due to the requested queue
>> depth.
>>
>> Looking at Barts results, it looks like it's usually fast, but sometimes
>> very slow. That would seem to indicate it's option #1 above that is the
>> issue. Bart, since this seems to wait for quite a bit, would it be
>> possible to cat the 'tags' file for that queue when it is stuck like
>> that?
>
> Hello Jens,
>
> Thanks for the assistance. Is this the output you were looking for ?
>
> # dmsetup table /dev/dm-1
> 0 256000 multipath 0 0 2 1 service-time 0 1 2 8:32 1 1 service-time 0 1
> 2 8:48 1 1
>
> # ls -ld /dev/sd[cd]
> brw-rw---- 1 root disk 8, 32 Jan  7 11:16 /dev/sdc
> brw-rw---- 1 root disk 8, 48 Jan  7 11:16 /dev/sdd
>
> # time mkfs.xfs -f /dev/dm-1 &
> [ ... ]
> real    4m12.101s
>
> # for d in sdc sdd; do echo ==== $d; (cd /sys/block/$d/mq &&
>    find|cut -c3-|grep tag|xargs grep -aH ''); done

You forgot dm-1, that's what mkfs is sleeping on. But given that sdc/sdd 
look idle, it still looks like a case of dm-1 not appropriately running 
the queues after insertion.

-- 
Jens Axboe