Re: [Qemu-devel] [PATCH 3/3] virtio-scsi-dataplane: Use main thread BH to set BDS' aio context

[Fam Zheng]

On Fri, 02/13 11:38, Paolo Bonzini wrote:
> 
> 
> On 13/02/2015 11:29, Fam Zheng wrote:
> > I think we should avoid duplicate everything on both virtio-blk and
> > virtio-scsi, so they will have differences.
> 
> True, but there are also similarities.  virtio-blk can also do per-VQ
> iothreads
> 
> > Why do you think Per VQ iothread is far away?
> 
> Because per-VQ iothread needs to use either fine-grained locks or,
> especially in the format and protocol driver, no locks at all.  No locks
> at all applies especially to the raw case, where we can more easily
> leverage kernel-side locks and thread-local storage.
> 
> Right now the lock is per-AioContext, but even if you made it
> BlockBackend-grained lock, the iothreads will just contend on it and
> each device won't get better performance than a single iothread.  Making
> a single backend faster is unfortunately an extremely important case; if
> you have multiple backends you can already move them to separate
> virtio-scsi controllers or virtio-blk devices.
> 
> We haven't even started thinking how the design should look like, so I
> think it's far away.
> 
> > Limiting to 1 thread for the
> > whole scsi bus doesn't sound ultimate solution for me. I think it's not 
> > harder
> > than the MMIO safety work we have, and also somehow independent to it.
> 
> I'm not sure it's independent.  While the MMIO safety work does not
> imply using fine-grained locks, it probably(*) implies using
> fine-grained critical sections.  Fine-grained critical sections are
> probably(**) a subset of the work needed for fine-grained locks or also
> for lockless operation.
> 
>       (*) probably = couldn't think of a better way
> 
>       (**) probably = I haven't even thought about it
> 
> Paolo
> 

OK, thanks for elaborating. I think for the sake of single IO thread support we
already started, the best option now is to go as you suggested - move
bdrv_set_aio_context to virtio_scsi_dataplane_start and hotplug callbacks.

Could you review patch 1? (And do we want patch 2?)

Thanks,
Fam