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RE: [Xen-devel] unnecessary VCPU migration happens again


> -----Original Message-----
> From: Emmanuel Ackaouy [mailto:ack@xxxxxxxxxxxxx] 
> Sent: 07 December 2006 10:38
> To: Xu, Anthony
> Cc: Petersson, Mats; xen-devel@xxxxxxxxxxxxxxxxxxx; xen-ia64-devel
> Subject: Re: [Xen-devel] unnecessary VCPU migration happens again
> On Thu, Dec 07, 2006 at 11:37:54AM +0800, Xu, Anthony wrote:
> > >From this logic, the migration happens frequently if the 
> numbers VCPU
> > is less than the number of logic CPU.
> This logic is designed to make better use of a partially idle
> system by spreading work across sockets and cores before
> co-scheduling them. It won't come into play if there are no
> idle execution units.
> Note that __csched_running_vcpu_is_stealable() will trigger a
> migration only when the end result would be strictly better
> than the current situation. Once the system is balanced, it
> will not bounce VCPUs around.
> > That I want to highlight is,
> > 
> > When HVM VCPU is executing IO operation,
> > This HVM VCPU is blocked by HV, until this IO operation
> > is emulated by Qemu. Then HV wakes up this HVM VCPU.
> > 
> > While PV VCPU will not be blocked by PV driver.
> > 
> > 
> > I can give below senario.
> > 
> > There are two sockets, two core per socket.
> > 
> > Assume, dom0 is running on socket1 core1,
> >  vti1 is runing on socket1 core2,
> > Vti 2 is runing on socket2 core1,
> > Socket2 core2 is idle.
> > 
> > If vti2 is blocked by IO operation, then socket2 core1 is idle,
> > That means two cores in socket2 are idle,
> > While dom0 and vti1 are running on two cores of socket1,
> > 
> > Then scheduler will try to spread dom0 and vti1 on these 
> two sockets.
> > Then migration happens. This is no necessary.
> Argueably, if 2 unrelated VCPUs are runnable on a dual socket
> host, it is useful to spread them across both sockets. This
> will give each VCPU more achievable bandwidth to memory.
> What I think you may be argueing here is that the scheduler
> is too aggressive in this action because the VCPU that blocked
> on socket 2 will wake up very shortly, negating the host-wide
> benefits of the migration when it does while still maintaining
> the costs.
> There is a tradeoff here. We could try being less aggressive
> in spreading stuff over idle sockets. It would be nice to do
> this with a greater understanding of the tradeoff though. Can
> you share more information, such as benchmark perf results,
> migration statistics, or scheduler traces?

I don't know if I've understood this right or not, but I believe the
penalty for switching from one core (or socket) to another is higher on
IA64 than on x86. I'm not an expert on IA64, but I remember someone at
the Xen Summit saying something to that effect - I think it was
something like executing a bunch of code to flush the TLB's or some

> Emmanuel.

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