Re: [PATCH v4 0/3] Support disabling fair tag sharing

[Bart Van Assche <bvanassche@acm.org>]

On 10/30/23 19:01, Yu Kuai wrote:
> I'm afraid that is not correct, fairness can't be guranteed at all, not
> even with just one scsi disk. This is because there are 8 wait queues in
> sbitmap, and threads are waiting in roundrobin mode, and each time
> wake_batch tags are released, wake_batch threads of one wait queue will
> be woke up, regardless that some threads can't grab tag after woken up,
> what's worse, thoese thread will be added to the tail of waitqueue
> again.
> 
> In the case that high io pressure under a slow disk, this behaviour will
> cause that io tail latency will be quite bad compared to sq from old
> kernel.
> 
> AFAIC, disable tag sharing will definitely case some regresion, for
> example, one disk will high io pressure, and another disk only issure
> one IO at a time, disable tag sharing can improve brandwith of fist
> disk, however, for the latter disk, IO latency will definitely be much
> worse.

Hi Yu,

All UFS users I know prefer that fair tag sharing is disabled for UFS
devices. So I propose to proceed with this patch series and additionally
to improve fair tag sharing for devices and transport layers that need
an improved sharing algorithm.

Thanks,

Bart.