[Qemu-devel] Re: [v1 PATCH 2/3]: Helper routines to use GLib threadpool infrastructure in 9pfs.

[Anthony Liguori <aliguori@us.ibm.com>]

On 03/16/2011 04:20 AM, Stefan Hajnoczi wrote:
> On Tue, Mar 15, 2011 at 1:13 PM, Anthony Liguori<aliguori@us.ibm.com>  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.

But realistically, the timer is firing at a high enough frequency that I 
doubt you'd even observe the latency.

There's an easy solution here, just do some sniff testing to see if you 
can tell the difference.  I bet you can't.

>    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.

We have probably 5 different ways to wake up a CPU.  I don't think we 
should add a new one just for this.

!CONFIG_IOTHREAD needs to go away in the very near future.  I'd rather 
focus on that.

Regards,

Anthony Liguori

> Stefan