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Date: Mon, 05 May 2008 21:48:31 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [4341] qemu: generate signals on tap I/O
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To: Anders <mail@flac.kalibalik.dk>
Cc: qemu-devel@nongnu.org

Anders wrote:
> Anthony Liguori wrote:
>> You're seeing improvement with normal QEMU?  Can you please post a 
>> description of what you're seeing improve with SIGIO.  SIGIO should 
>> really only slow things down.
>>   
>
> I found this issue in KVM. I have not timed in QEMU, but the issue 
> seemed generic, so I posted it upstream.

Right, this was an artifact of how KVM handled select.  It's not generic 
at all.

> My usecase was a new VNC connection delaying until the next timer tick 
> (which is a really long time in some KVM configurations).
>
> The improvement is from running the select() immediately, rather than 
> waiting around for a timer to expire.

In KVM today, select() is in it's own thread so it will service IO 
immediately.  This makes SIGIO unnecessary (not quite correct yet but 
will be very soon).

Regards,

Anthony Liguori

> Thanks,
> Anders.
>