[Qemu-devel] Re: [RFC] Moving the kvm ioapic, pic, and pit back to userspace

["David S. Ahern" <daahern@cisco.com>]

On 06/07/10 12:46, Avi Kivity wrote:
> On 06/07/2010 07:31 PM, David S. Ahern wrote:
>>
>> On 06/07/10 09:26, Avi Kivity wrote:
>>
>>   
>>> The original motivation for moving the PIC and IOAPIC into the kernel
>>> was performance, especially for assigned devices.  Both devices are high
>>> interaction since they deal with interrupts; practically after every
>>> interrupt there is either a PIC ioport write, or an APIC bus message,
>>> both signalling an EOI operation.  Moving the PIT into the kernel
>>> allowed us to catch up with missed timer interrupt injections, and
>>> speeded up guests which read the PIT counters (e.g. tickless guests).
>>>
>>> However, modern guests running on modern qemu use MSI extensively; both
>>> virtio and assigned devices now have MSI support; and the planned VFIO
>>> only supports kernel delivery via MSI anyway; line based interrupts will
>>> need to be mediated by userspace.
>>>      
>> The "modern" guest comment is a bit concerning. 2.4 kernels (e.g.,
>> RHEL3) use the PIT for timekeeping and will still be around for a while.
>> RHEL4 and RHEL5 will be around for a long time to come. Not sure how
>> those fit within the "modern" label, though I see my RHEL4 guest is
>> using the pit as a timesource.
>>    
> 
> First of all, the existing code will remain for a long while (several
> years).  We still have to support existing userspace.
> 
> But, that's not a satisfactory answer.  I don't want users to choose
> which device model to use according to their guest.  As far as I'm
> concerned all guests are triple-boot with the guest rebooting to a
> different OS every half hour.
> 
> 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. 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.

David