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Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy lo

From: Gleb Natapov
Subject: Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load.
Date: Sat, 8 Nov 2008 10:36:20 +0200

On Thu, Nov 06, 2008 at 09:37:56AM -0600, Anthony Liguori wrote:
> Gleb Natapov wrote:
>> On Thu, Nov 06, 2008 at 08:40:09AM -0600, Anthony Liguori wrote:
>>> Gleb: are you perhaps using a qcow2 file in conjunction with 
>>> -snapshot?       
>> I am using qcow2, but without -snapshot.
> Okay, you would still see this if your qcow2 is relatively small  
> compared to the possible size it could be.
> I totally believe that you could miss ticks from qcow2 metadata writing  
> even with 100hz clock especially since we're using O_SYNC.  A relatively  
> large write that has to extend the qcow2 file multiple times could  
> conceivably block the guest for more than 10ms.  However, this is a bug  
> in qcow2 IMHO.  Metadata updates should be done asynchronously and if  
> they did, I bet this problem wouldn't occur.  A test against raw should  
> confirm this.
I ran the copy test once again with qcow2 image, but this time I copied 
from qcow2 to network fs and the drift still exists. Much smaller
though. 8 second per hour AFAIR.

>>>> If part of qemu gets swapped out then all bets are off, and you can 
>>>> easily stall for significant fractions of a second. No amount of 
>>>> host high resolution time support will help you there.
>>> Running a steady workload, you aren't going to be partially swapped.
>> We want to oversubscribe host as much as possible, and workload will
>> vary during a lifetime of the VMs.
> I understand that we want guest time behave even when we're  
> overcommitting the host CPU.
> However, let's make sure we understand exactly what's going on such that  
> we know precisely what we're fixing.  I believe the file copy benchmark  
> is going to turn out to no longer produce drift with a raw image.  If  
> that's the case, you'll need to find another benchmark to quantify drift.
Yes indeed. With raw image copy benchmark no longer runs enough time to
produce time drift big enough to be visible. So I ran this disk test
for ~12 hours and the time drift was 12 secs (if I weren't so lazy and
wrote bat file to copy c:\windows in a loop I am sure result would be the
same). This is on completely idle host.

> I think the best ones are going to be intense host workload (and let's  
> see how much is needed before we start drifting badly) and high guest  
> frequencies with hosts that lack high resolution timers.  I think with a  
> high resolution guest and no host overcommit, it should be very  
> difficult to produce drift regardless of what the guest is doing.
Later I'll try to generate load on a host an see how this affects
guest's time drift.


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