Re: [PATCH V4] virtio-fs: Improved request latencies when Virtio queue is full

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jul 03, 2023 at 09:14:59PM +0200, Peter-Jan Gootzen wrote:
> When the Virtio queue is full, a work item is scheduled
> to execute in 1ms that retries adding the request to the queue.
> This is a large amount of time on the scale on which a
> virtio-fs device can operate. When using a DPU this is around
> 40us baseline without going to a remote server (4k, QD=1).
> This patch queues requests when the Virtio queue is full,
> and when a completed request is taken off, immediately fills
> it back up with queued requests.
> 
> This reduces the 99.9th percentile latencies in our tests by
> 60x and slightly increases the overall throughput, when using a
> queue depth 2x the size of the Virtio queue size, with a
> DPU-powered virtio-fs device.
> 
> Signed-off-by: Peter-Jan Gootzen <peter-jan@gootzen.net>
> ---
> V4: Removed return value on error changes to simplify patch,
> that should be changed in another patch.
> V3: Fixed requests falling into the void when -ENOMEM and no new
> incoming requests. Virtio-fs now always lets -ENOMEM bubble up to
> userspace. Also made queue full condition more explicit with
> -ENOSPC in `send_forget_request`.
> V2: Not scheduling dispatch work anymore when not needed
> and changed delayed_work structs to work_struct structs
> 
>  fs/fuse/virtio_fs.c | 32 +++++++++++++++++---------------
>  1 file changed, 17 insertions(+), 15 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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