Re: virtio-fs: adding support for multi-queue

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Feb 08, 2023 at 05:29:25PM +0100, Peter-Jan Gootzen wrote:
> On 08/02/2023 11:43, Stefan Hajnoczi wrote:
> > On Wed, Feb 08, 2023 at 09:33:33AM +0100, Peter-Jan Gootzen wrote:
> > > 
> > > 
> > > On 07/02/2023 22:57, Vivek Goyal wrote:
> > > > On Tue, Feb 07, 2023 at 04:32:02PM -0500, Stefan Hajnoczi wrote:
> > > > > On Tue, Feb 07, 2023 at 02:53:58PM -0500, Vivek Goyal wrote:
> > > > > > On Tue, Feb 07, 2023 at 02:45:39PM -0500, Stefan Hajnoczi wrote:
> > > > > > > On Tue, Feb 07, 2023 at 11:14:46AM +0100, Peter-Jan Gootzen wrote:
> > > > > > > > Hi,
> > > > > > > > 
> > > > > > 
> > > > > > [cc German]
> > > > > > 
> > > > > > > > For my MSc thesis project in collaboration with IBM
> > > > > > > > (https://github.com/IBM/dpu-virtio-fs) we are looking to improve the
> > > > > > > > performance of the virtio-fs driver in high throughput scenarios. We think
> > > > > > > > the main bottleneck is the fact that the virtio-fs driver does not support
> > > > > > > > multi-queue (while the spec does). A big factor in this is that our setup on
> > > > > > > > the virtio-fs device-side (a DPU) does not easily allow multiple cores to
> > > > > > > > tend to a single virtio queue.
> > > > > > 
> > > > > > This is an interesting limitation in DPU.
> > > > > 
> > > > > Virtqueues are single-consumer queues anyway. Sharing them between
> > > > > multiple threads would be expensive. I think using multiqueue is natural
> > > > > and not specific to DPUs.
> > > > 
> > > > Can we create multiple threads (a thread pool) on DPU and let these
> > > > threads process requests in parallel (While there is only one virt
> > > > queue).
> > > > 
> > > > So this is what we had done in virtiofsd. One thread is dedicated to
> > > > pull the requests from virt queue and then pass the request to thread
> > > > pool to process it. And that seems to help with performance in
> > > > certain cases.
> > > > 
> > > > Is that possible on DPU? That itself can give a nice performance
> > > > boost for certain workloads without having to implement multiqueue
> > > > actually.
> > > > 
> > > > Just curious. I am not opposed to the idea of multiqueue. I am
> > > > just curious about the kind of performance gain (if any) it can
> > > > provide. And will this be helpful for rust virtiofsd running on
> > > > host as well?
> > > > 
> > > > Thanks
> > > > Vivek
> > > > 
> > > There is technically nothing preventing us from consuming a single queue on
> > > multiple cores, however our current Virtio implementation (DPU-side) is set
> > > up with the assumption that you should never want to do that (concurrency
> > > mayham around the Virtqueues and the DMAs). So instead of putting all the
> > > work into reworking the implementation to support that and still incur the
> > > big overhead, we see it more fitting to amend the virtio-fs driver with
> > > multi-queue support.
> > > 
> > > 
> > > > Is it just a theory at this point of time or have you implemented
> > > > it and seeing significant performance benefit with multiqueue?
> > > 
> > > It is a theory, but we are currently seeing that using the single request
> > > queue, the single core attending to that queue on the DPU is reasonably
> > > close to being fully saturated.
> > > 
> > > > And will this be helpful for rust virtiofsd running on
> > > > host as well?
> > > 
> > > I figure this would be dependent on the workload and the users-needs.
> > > Having many cores concurrently pulling on their own virtq and then
> > > immediately process the request locally would of course improve performance.
> > > But we are offloading all this work to the DPU, for providing
> > > high-throughput cloud services.
> > 
> > I think Vivek is getting at whether your code processes requests
> > sequentially or in parallel. A single thread processing the virtqueue
> > that hands off requests to worker threads or uses io_uring to perform
> > I/O asynchronously will perform differently from a single thread that
> > processes requests sequentially in a blocking fashion. Multiqueue is not
> > necessary for parallelism, but the single queue might become a
> > bottleneck.
> 
> Requests are handled non-blocking with remote IO on the DPU. Our current
> architecture is as follows:
> T1: Tends to the Virtq, parses FUSE to remote IO and fires off the
> asynchronous remote IO.
> T2: Polls for completion on the remote IO and parses it back to FUSE, puts
> the FUSE buffers in a completion queue of T1.
> T1: Handles the Virtio completion and DMA of the requests in the CQ.
> 
> Thread 1 is busy polling on its two queues (Virtq and CQ) with equal
> priority, thread 2 is busy polling as well. This setup is not really
> optimal, but we are working within the constraints of both our DPU and
> remote IO stack.

Why does T1 need to handle VIRTIO completion and DMA requests instead of
T2?

Stefan

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