Re: [PATCH 2/2] fuse: virtiofs: Add basic multiqueue support

[Chirantan Ekbote <chirantan@chromium.org>]

On Sat, May 2, 2020 at 12:48 AM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Fri, May 01, 2020 at 04:14:38PM +0900, Chirantan Ekbote wrote:
> > On Tue, Apr 28, 2020 at 12:20 AM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > Instead of modifying the guest driver, please implement request
> > > parallelism in your device implementation.
> >
> > Yes, we have tried this already [1][2].  As I mentioned above, having
> > additional threads in the server actually made performance worse.  My
> > theory is that when the device only has 2 cpus, having additional
> > threads on the host that need cpu time ends up taking time away from
> > the guest vcpu.  We're now looking at switching to io_uring so that we
> > can submit multiple requests from a single thread.
>
> The host has 2 CPUs?  How many vCPUs does the guest have?  What is the
> physical storage device?  What is the host file system?

The host has 2 cpus.  The guest has 1 vcpu.  The physical storage
device is an internal ssd.  The file system is ext4 with directory
encryption.


>
> io_uring's vocabulary is expanding.  It can now do openat2(2), close(2),
> statx(2), but not mkdir(2), unlink(2), rename(2), etc.
>
> I guess there are two options:
> 1. Fall back to threads for FUSE operations that cannot yet be done via
>    io_uring.
> 2. Process FUSE operations that cannot be done via io_uring
>    synchronously.
>

I'm hoping that using io_uring for just the reads and writes should
give us a big enough improvement that we can do the rest of the
operations synchronously.

Chirantan