Re: Why is MEGASAS_SAS_QD set to 256?

[Yu Kuai]

Hi,

在 2022/11/27 17:42, Ming Lei 写道:
> 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.

Yes, io can still be merged by plug. I just find it a little werid to
set default elevator as deadline, and default queue_depth to 256. Which
means deadline here is useless.

> > > 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.

Yes, I'm testing with multiple SSDs / NVMEs, and I can't get optimal
performance by default.

Thanks,
Kuai
> Thanks
> Ming
>
> .