On Thu, Nov 02, 2023 at 03:10:52PM +0100, Kevin Wolf wrote:
> 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: [email protected] ("[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?"[PATCH v3 5/5] block-coroutine-wrapper: use qemu_get_current_aio_context()" is necessary so that virtio_blk_sect_range_ok -> blk_get_geometry -> blk_nb_sectors -> bdrv_refresh_total_sectors -> bdrv_poll_co can be called without holding the AioContext lock. That case only happens when the BlockDriverState is a file-posix host CD-ROM or a file-win32 host_device. Most users will never hit this problem, but it would be unsafe to proceed merging code without this patch. > 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/[email protected]/ > > Is this right? Yes, it depends on "[PATCH 0/4] virtio-blk: prepare for the multi-queue block layer" so that every AioContext is able to handle Linux AIO or io_uring I/O and to stop using the AioContext lock in the virtio-blk I/O code path. Stefan > > Given that soft freeze is early next week, maybe we should try to merge > just the bare minimum of strictly necessary dependencies. > > Kevin >
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