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[Qemu-devel] Re: [v1 PATCH 2/3]: Helper routines to use GLib threadpool

From: Stefan Hajnoczi
Subject: [Qemu-devel] Re: [v1 PATCH 2/3]: Helper routines to use GLib threadpool infrastructure in 9pfs.
Date: Wed, 16 Mar 2011 09:20:45 +0000

On Tue, Mar 15, 2011 at 1:13 PM, Anthony Liguori <address@hidden> wrote:
> Why even bothering signaling for completion with the virtio-9p threadpool?
> There's no sane guest that's going to poll on virtio-9p completion with
> interrupts disabled and no timer.  Once we enable the I/O thread by default,
> it won't even be necessary for the paio layer.

It's not just about preventing livelock under extreme cases.  If you
omit the signal then !CONFIG_IOTHREAD builds will see increased
latency because virtfs request completion will piggy-back on other
events that *do* interrupt the vcpu.  I'm no fan of !CONFIG_IOTHREAD
but skipping signals is currently bad practice and will continue to be
until CONFIG_IOTHREAD is removed entirely.

The proper solution would be a thin abstraction for thread-safe
notification that compiles out signals when CONFIG_IOTHREAD is used.
Then we have one place in QEMU that does signalling correctly and we
can optimize it or remove CONFIG_IOTHREAD completely without having
the burden of duplicating this code in several places.


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