nvme: split bios issued in reverse order

[Jonathan Nicklin]

On Tue, May 24, 2022 at 4:29 PM Keith Busch <kbusch at kernel.org> wrote:
>
> On Tue, May 24, 2022 at 03:37:56PM -0400, Jonathan Nicklin wrote:
> > On Tue, May 24, 2022 at 3:25 PM Keith Busch <kbusch at kernel.org> wrote:
> > >
> > > On Tue, May 24, 2022 at 12:12:29PM -0400, Jonathan Nicklin wrote:
> > > >
> > > > The command lines you have are for read operations. The behavior seems
> > > > only to appear with writes.
> > >
> > > Huh, I'll be darn...
> > >
> > > I think it's because the writes are plugged and the reads are not. The plug
> > > appends requests to the head of the plug list, and unplugging will dispatch the
> > > requests in the reverse order.
> >
> > Thanks for confirming! That's about where I got to. Do you have any
> > ideas on what might explain the difference in behavior between
> > fio/pvsync and fio/libaio? And, why does this not seem to occur when
> > only one nvme queue is present? Perhaps the in-order cases are an
> > indication of not being plugged?
>
> I actually didn't see a difference between libaio or psync, and also io_uring.
> They all plugged and reversed the dispatch order. Do you have a scheduler
> enabled?

Nope, there's no scheduler in the way.