Re: Why is MEGASAS_SAS_QD set to 256?

[Ming Lei]

On Sat, Nov 26, 2022 at 02:08:02PM +0800, Yu Kuai wrote:
> Hi, Ming
> 
> 在 2022/11/26 10:18, Ming Lei 写道:
> > 
> > If you want aggressive merge on sequential IO workload, the queue depth need
> > to be a bit less, then more requests can be staggered into scheduler queue,
> > and merge chance is increased.
> 
> But if nr_requests >= queue_depth, it seems to me elevator will have no
> effect, no request can be merged or sorted by scheduler, right?

Yeah.

If nr_requests <= queue_depth, every request can be queued to
driver/device, so requests won't be merged by scheduler.

But plug merge still works if IOs are submitted as batch.

> > 
> > If you want good perf on random IO perf, the queue depth needs to
> > be deep enough to have enough parallelism for saturating SSD internal.
> > 
> > But we don't recognize sequential/random IO pattern, and usually fixed
> > queue depth is used.
> 
> Is it possible to use none elevator and set large queue_depth if nvme is
> used in this case?

Yeah, if the storage is SSD, usually none with bigger queue_depth should
help, and usually 256 should be enough to saturate one single SSD for
one well implemented driver.


Thanks
Ming