Re: [PATCH V3] virtio-fs: Improved request latencies when Virtio queue is full

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Jun 02, 2023 at 03:32:26PM +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.
> 
> Furthermore, the virtio-fs driver now also always lets -ENOMEM
> errors go to userspace instead of retrying the request in the
> driver.
> 
> Signed-off-by: Peter-Jan Gootzen <peter-jan@gootzen.net>
> ---
> 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

Hi Peter-Jan,
I will be traveling and will try to look at this patch as soon as
possible.

If Vivek and others are happy, please don't wait for me.

Thanks,
Stefan

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