Re: [PATCH RFC v2 0/8] blk-mq: improve tag fair sharing

[Yu Kuai]

Hi,

在 2023/10/23 12:38, Ming Lei 写道:
> Hello Yu Kuai,
>
> On Sat, Oct 21, 2023 at 11:47:58PM +0800, Yu Kuai wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > Current implementation:
> >   - a counter active_queues record how many queue/hctx is sharing tags,
> >   and it's updated while issue new IO, and cleared in
> >   blk_mq_timeout_work().
> >   - if active_queues is more than 1, then tags is fair shared to each
> >   node;
>
> Can you explain a bit what the problem is in current tag sharing?
> And what is your basic approach for this problem?
>
> Just mentioning the implementation is not too helpful for initial
> review, cause the problem and approach(correctness) need to be
> understood first.

Of course, I'll add following if there will be a v3;

Current problems:

If there are multiple active_queues, then tag is fair shared to each
queue, and if one queue is not busy(for example, only issue one IO once
for a while), then shared tags for this queue is wasted and can't be
used for other queues.

Depends on the hardware, this might casue performance problems in some
user case. For example, as reported by [1], UFS devices
have multiple logical units. One of these logical units (WLUN) is used
to submit control commands, e.g. START STOP UNIT. If any request is
submitted to the WLUN, the queue depth is reduced from 31 to 15 or
lower for data LUNs.

This patchset first delay tag sharing from issue IO to failed to get
driver tag; then add a counter to record how many times shared queue
failed to get driver tag to indicate if the queue is busy; finially,
allow busy queue to borrow more tags from idle queue.

Thanks,
Kuai

> Thanks,
> Ming
>
> .