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

["Venkateswararao Jujjuri (JV)" <jvrao@linux.vnet.ibm.com>]

On 3/16/2011 6:10 AM, Anthony Liguori wrote:
> 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.

If that is the case, how about making VirtFS dependent on CONFIG_IOTHREAD
during the configuration? This can help even if !CONFIG_IOTHREAD lingers around
for some more time and would avoid any of the Stefan's concerns.

- JV

> 
> Regards,
> 
> Anthony Liguori
> 
>> Stefan
>