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Subject: [Qemu-devel] Re: [RFC] Moving the kvm ioapic, pic,
	and pit back to userspace
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To: "David S. Ahern" <daahern@cisco.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, KVM list <kvm@vger.kernel.org>

On 06/07/2010 09:54 PM, David S. Ahern wrote:
>
>> So it's important to know how often your RHEL3/4 guest queries the PIT
>> (not just receives interrupts, actually reads the counter) under a
>> realistic load.  If you have such a number (in reads/sec) that would be
>> a good input to this discussion.
>>
>>      
> Aps that invoke gettimeofday a lot.

Ask a stupid question, get an "it depends on the workload" answer.

> As I recall RHEL3 uses the TSC
> between timer interrupts, but RHEL4 samples counters on each
> gettimeofday call:
>
> http://www.mail-archive.com/kvm@vger.kernel.org/msg07231.html
>
> Because of that performance of applications that timestamp log entries
> (like a certain product I work on) takes a hit on KVM unless the TSC is
> the clock source.
>    

So it looks like dropping the PIT out of the kernel, let alone the 
PIC/IOAPIC, is out of the question.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.