Re: [PATCH v2 0/2] virtio-blk: add iothread-vq-mapping parameter

[Kevin Wolf]

Am 18.09.2023 um 18:16 hat Stefan Hajnoczi geschrieben:
> virtio-blk and virtio-scsi devices need a way to specify the mapping between
> IOThreads and virtqueues. At the moment all virtqueues are assigned to a 
> single
> IOThread or the main loop. This single thread can be a CPU bottleneck, so it 
> is
> necessary to allow finer-grained assignment to spread the load. With this
> series applied, "pidstat -t 1" shows that guests with -smp 2 or higher are 
> able
> to exploit multiple IOThreads.
> 
> This series introduces command-line syntax for the new iothread-vq-mapping
> property is as follows:
> 
>   --device 
> '{"driver":"virtio-blk-pci","iothread-vq-mapping":[{"iothread":"iothread0","vqs":[0,1,2]},...]},...'
> 
> IOThreads are specified by name and virtqueues are specified by 0-based
> index.
> 
> It will be common to simply assign virtqueues round-robin across a set
> of IOThreads. A convenient syntax that does not require specifying
> individual virtqueue indices is available:
> 
>   --device 
> '{"driver":"virtio-blk-pci","iothread-vq-mapping":[{"iothread":"iothread0"},{"iothread":"iothread1"},...]},...'
> 
> There is no way to reassign virtqueues at runtime and I expect that to be a
> very rare requirement.
> 
> Note that JSON --device syntax is required for the iothread-vq-mapping
> parameter because it's non-scalar.
> 
> Based-on: 20230912231037.826804-1-stefanha@redhat.com ("[PATCH v3 0/5] 
> block-backend: process I/O in the current AioContext")

Does this strictly depend on patch 5/5 of that series, or would it just
be a missed opportunity for optimisation by unnecessarily running some
requests from a different thread?

I suspect it does depend on the other virtio-blk series, though:

[PATCH 0/4] virtio-blk: prepare for the multi-queue block layer
https://patchew.org/QEMU/20230914140101.1065008-1-stefanha@redhat.com/

Is this right?

Given that soft freeze is early next week, maybe we should try to merge
just the bare minimum of strictly necessary dependencies.

Kevin