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Re: [Qemu-block] [Qemu-devel] [PATCH 00/18] dataplane: remove RFifoLock

From: Paolo Bonzini
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 00/18] dataplane: remove RFifoLock (including almost all previously sent patches)
Date: Mon, 17 Oct 2016 05:17:58 -0400 (EDT)

> On 10/13/2016 07:34 PM, Paolo Bonzini wrote:
> > This patch reorganizes aio_poll callers to establish new rules for
> > dataplane locking.  The idea is that I/O operations on a dataplane
> > BDS (i.e. one where the AioContext is not the main one) do not call
> > aio_poll anymore.  Instead, they wait for the operation to end in the
> > other I/O thread, at which point the other I/O thread calls bdrv_wakeup
> > to wake up the main thread.
> > 
> > With this change, only one thread runs aio_poll for an AioContext.
> > While aio_context_acquire/release is still needed to protect the BDSes,
> > it need not interrupt the other thread's event loop anymore, and therefore
> > it does not need contention callbacks anymore.  Thus the patch can remove
> > RFifoLock.  This fixes possible hangs in bdrv_drain_all, reproducible (for
> > example) by unplugging a virtio-scsi-dataplane device while there is I/O
> > going on for a virtio-blk-dataplane on the same I/O thread.
> Have you seen improvements or deteriorations in performance with single disks
> or multiple disks? Is there a branch?

I will prepare a branch for v2.

This should have no performance effect.  The affected code only runs during QMP
operations such as live snapshots or completing a block job.


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