[Virtio-fs] [PATCH 2/2] fuse: virtiofs: Add basic multiqueue support

[Stefan Hajnoczi]

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 at redhat.com> wrote:
> > On Fri, Apr 24, 2020 at 03:25:40PM +0900, Chirantan Ekbote wrote:
> > Even if you don't care about SMP performance, using multiqueue as a
> > workaround for missing request parallelism still won't yield the best
> > results.  The guest should be able to submit up to the maximum queue
> > depth of the physical storage device.  Many Linux block drivers have max
> > queue depths of 64.  This would require 64 virtqueues (plus the queue
> > selection algorithm would have to utilize each one) and shows how
> > wasteful this approach is.
> >
> 
> I understand this but in practice unlike the virtio-blk workload,
> which is nothing but reads and writes to a single file, the virtio-fs
> workload tends to mix a bunch of metadata operations with data
> transfers.  The metadata operations should be mostly handled out of
> the host's file cache so it's unlikely virtio-fs would really be able
> to fully utilize the underlying storage short of reading or writing a
> really huge file.

I agree that a proportion of heavy I/O workloads on virtio-blk become
heavy metadata I/O workloads on virtio-fs.

However, workloads consisting mostly of READ, WRITE, and FLUSH
operations still exist on virtio-fs.  Databases, audio/video file
streaming, etc are bottlenecked on I/O performance.  They need to
perform well and virtio-fs should strive to do that.

> > 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?

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.

Stefan
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