[Bug 1341195] Re: KVM guests getting slow by time

[Serge Hallyn]

Quoting Tamas Papp (1341195 at bugs.launchpad.net):
> On 07/16/2014 06:35 PM, Serge Hallyn wrote:
> 
> 
> > To be clear, you have not yet gotten the bug since you've set
> > KSM_ENABLED to 0, or have you?
> 
> I just did it, so no, not yet.
> 
> This happened by time:
> 
> Saucy (everything is fine) -> Trusty (issue happened) -> Trusty with 
> Saucy kernel 3 weeks ago (no issue) -> today with 3 weeks uptime (issue 
> happened) -> KSM_ENABLED=0 + reboot with Trusty kernel -> now
> 
> I just want to be clear, that both of us understand the same on this.
> 
> 
> > Interesting about the saucy kernel.  Ideally we could reproduce this
> > reliably enough to really bisect.
> >
> > I wonder if we can reproduce without qemu.
> 
> 
> I have a some other servers with LXC with no issue, If there is KVM, 
> then issue happens.
> Does it really not show up anywhere else?

I've personally not seen it, and noone on my team, who use a lot of
kvm instances, has seen it.  Our two current theories are that (a) it
has to do with ksm page migration across numa-nodes, or (b) it has to
do with a race with transparent hugepages versus ksm.  For (b) there is
already a commit in Linus' tree (f72e7dcd).  For (a), you can probably
test by setting /sys/kernel/mm/ksm/merge_across_nodes to 0.  (We are
trying these as well, but as the testcases are not 100% positive
it'd be good to have you try as well)

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https://bugs.launchpad.net/bugs/1341195

Title:
  KVM guests getting slow by time

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