On 5/12/23 20:09, Yu Kuai wrote:
在 2023/05/13 2:12, Bart Van Assche 写道:
The fair tag sharing algorithm has a negative impact on all SCSI
devices with multiple logical units. This is because logical units are
considered active until (request timeout) seconds have elapsed after
the logical unit stopped being used (see also the blk_mq_tag_idle()
call in blk_mq_timeout_work()). UFS users are hit by this because UFS
3.0 devices have a limited queue depth (32) and because power
management commands are submitted to a logical unit (WLUN). Hence, it
happens often that the block layer "active queue" counter is equal to
2 while only one logical unit is being used actively (a logical unit
backed by NAND flash). The performance difference between queue depths
16 and 32 for UFS devices is significant.
We meet similiar problem before, but I think remove tag fair sharing
might cause some problems, because get tag is not fair currently, for
example 2 devices share 32 tag, while device a issue large amount of
io concurrently, and device b only issue one io, in this case, if fair
tag sharing is removed, device b can get bad io latency.
By the way, I tried to propose a way to workaround this by following:
1) disable fair tag sharing untill get tag found no tag is avaiable;
2) enable fair tag sharing again if the disk donesn't faild to get tag
for a period of time;
Can this approch be considered?
I'm afraid that this approach won't help for the UFS driver since it is
likely that all tags are in use by a single logical unit during an IOPS
test. Hence, fair sharing would be enabled even when we don't want it to
be enabled.
I propose that we switch to one of these two approaches:
* Either remove the fair tag sharing code entirely and rely on the
fairness mechanism provided by the sbitmap code. I'm referring to how
__sbitmap_queue_wake_up() uses the wake_index member variable.
* Or make the behavior of the fairness algorithm configurable from user
space. One possible approach is to make the proportion of tags for a
logical unit / NVMe namespace configurable via sysfs. This will allow to
reduce the number of tags for the WLUN of UFS devices.
Thanks,
Bart.
