Re: [PATCH v1 0/5] dm: dm-user: New target that proxies BIOs to userspace

[Palmer Dabbelt]

On Tue, 22 Dec 2020 06:36:16 PST (-0800), snitzer@xxxxxxxxxx wrote:
> On Tue, Dec 22 2020 at  8:32am -0500,
> Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> > On Mon, Dec 14, 2020 at 07:00:57PM -0800, Palmer Dabbelt wrote:
> > > I haven't gotten a whole lot of feedback, so I'm inclined to at least have some
> > > reasonable performance numbers before bothering with a v2.
> >
> > FYI, my other main worry beside duplicating nbd is that device mapper
> > really is a stacked interface that sits on top of other block device.
> > Turning this into something else that just pipes data to userspace
> > seems very strange.
>
> I agree.  Only way I'd be interested is if it somehow tackled enabling
> much more efficient IO.  Earlier discussion in this thread mentioned
> that zero-copy and low overhead wasn't a priority (because it is hard,
> etc).  But the hard work has already been done with io_uring.  If
> dm-user had a prereq of leaning heavily on io_uring and also enabled IO
> polling for bio-based then there may be a win to supporting it.
>
> But unless lower latency (or some other more significant win) is made
> possible I just don't care to prop up an unnatural DM bolt-on.

I don't remember if I mentioned this in the thread, but it was definately in
the Plumbers talk, but I'd had the general idea bouncing around that it would
be possible to write a high-performance version of this using an interface
similar to the one provided here while relying on io_uring for the
high-performance userspace.  That definately won't work with exactly the
current interface, but my hope was to avoid writing my own high-performance
ring buffer.  My worry was that it'll be too tricky to map this all to
zero-copy, and I guess I forgot about it.

Now that you bring it up, it certainly seems worth taking a shot at.  We'd
essentially have the best of both worlds: userspace implementations that want
to be simple could just use read()/write(), while those that want to be higher
performance could have their implicit ring buffer.

I'm currently trying to put together a benchmarking setup that is of sufficient
fidelity that I would believe the numbers, which is really why I don't have any
performance numbers yet (no sense posting numbers I would shoot down :)).  I'll
try to remember to take a shot at an io_uring based userspace (probably with
some dm-user interface modifications) to see how it feels.