Re: [PATCH 0/6] In kernel PIT patch

[Dor Laor <dor.laor@gmail.com>]

On Tue, 2008-03-04 at 18:50 -0600, Anthony Liguori wrote:
> Dor Laor wrote:
> > On Tue, 2008-03-04 at 09:52 -0600, Anthony Liguori wrote:
> >   
> >> Yang, Sheng wrote:
> >>     
> >>> Hi
> >>>
> >>> Here is the last in-kernel PIT patch for KVM. The mainly change from last 
> >>> version is the supporting to save/restore. I also tested live migration.
> >>>
> >>> The other modifies including some date structure changed to be better for 
> >>> supporting the save/restore. I moved the PIT timer to outside of channel 
> >>> structure, which explicitly means only one channel (channel 0) would trigger 
> >>> it.
> >>>
> >>> After fix TSC problem on SMP PAE RHEL5/5.1 guest, now the patch works well 
> >>> without any modify of kernel parameter.
> >>>   
> >>>       
> >> How are you measuring the improvements from an in-kernel PIT?  From your 
> >> mails, you're claiming it increases the timer accuracy.  How are you 
> >> measuring it and how much does it improve it?
> >>
> >>     
> >
> > It's also a functionality addition: userspace pit & pic combination
> > needed to use -tdf option (time drift fix). The tdf took care of pending
> > pit irqs and tried to make the guest ack the right number of irqs the
> > pit was configured.
> >   
> 
> I thought there was some discussion about whether -tdf was every useful 
> in practice?

It works.
Just try to play a movie in windows standard HAL with and w/o -tdf
--no-irq-chip and you'll see the difference.

> 
> > Once we switched to the default in-kernel pic, the userspace pit
> > couldn't get the acks from the pit.
> > One can see the effect when running multiple guests (windows, standard
> > HAL) playing video, the time slows down.
> >   
> 
> Okay, that makes sense.  So have you done any tests to confirm this?  We 
> suffered through a fair number of regressions when we moved to an 
> in-kernel APIC.  Before moving another big chunk of code in the kernel 
> and going through possible regressions, I want to make sure we have a 
> measurable argument that it's the right thing to do.
> 
> So how do we measure the benefits of an in-kernel PIT?

Play the same movie using the kernel's pit.

> 
> Regards,
> 
> Anthony Liguori
> 
> > This patch set has a pending counter and takes care for it too.
> >
> >   
> >> Do you expect an overall performance improvement from this or is it 
> >> simply about improving timer accuracy?
> >>
> >>     
> >
> > It will probably help older kernels with slow HZ run faster HZ guests.
> > Without CONFIG_DYNTICK the guests behaved jumpy because of that.
> >
> >   
> >> Regards,
> >>
> >> Anthony Liguori
> >>
> >>
> >>
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> >
> >   
> 


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