7 messages in com.xensource.lists.xen-devel[Xen-devel] RE: A credit scheduler issue
FromSent OnAttachments
Kamble, Nitin A29 Jun 2006 18:13 
Keir Fraser30 Jun 2006 00:05 
Emmanuel Ackaouy30 Jun 2006 03:45 
Kamble, Nitin A30 Jun 2006 12:23 
Ian Pratt30 Jun 2006 13:22 
Emmanuel Ackaouy30 Jun 2006 13:28 
Li, Xin B06 Jul 2006 03:50 
Subject:[Xen-devel] RE: A credit scheduler issue
From:Kamble, Nitin A (niti@intel.com)
Date:06/30/2006 12:23:24 PM
List:com.xensource.lists.xen-devel

Keir, Emmanuel, Thanks for the detailed answers, and your views. I agree that my small change should not affect correctness. I didn't see the migration often, I saw the dom0 vcpus migrations happening 4, 5 times from boot to start of xend. I think we should avoid these migrations; why waste the cache hotness? How solid is the credit scheduler now for DomUs on a SMP box? On 32bit, PAE & 64bit? It would be a useful data point for me to debug the HVM guest issues with the credit scheduler.

Thanks & Regards, Nitin

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----------- Open Source Technology Center, Intel Corp

-----Original Message----- From: Emmanuel Ackaouy [mailto:ac@xensource.com] Sent: Friday, June 30, 2006 3:46 AM To: Kamble, Nitin A Cc: xen-@lists.xensource.com; Keir Fraser; Ian Pratt Subject: Re: A credit scheduler issue

Hi Nitin,

On Thu, Jun 29, 2006 at 06:13:51PM -0700, Kamble, Nitin A wrote:

I am trying to debug the credit scheduler to solve the many

HVM

domain

instability issues we have found with the credit scheduler.

Great. As Keir pointed out though the problems you are seeing may not actually be in the credit scheduler itself.

While debugging I notice an odd behavior; When running on a 2 CPU system, dom0 gets 2 vcpus by default. And even if there are no other domains running in the system, the dom0 vcpus are getting migrated to different pcpus in the load balance. I think it is due to the preemption happening in the credit scheduler; and it is not necessary and is actually wasteful to move vcpus when no of vcpus in the system are equal to

no

of

pcpus.

I would like to know your thinking about this behavior. Is it an intended in the design?

This should be very rare. If a VCPU were woken up and put on the runq of an idle CPU, a peer physical CPU that is in the scheduler code at that exact time could potentially pick up the just woken up VCPU.

We can do things to shorten this window, like not pick up a VCPU from a remote CPU that is currently idle and therefore probably racing with us to run said newly woken up VCPU on its runq. But I'm not sure this happens frequently enough to warrant the added complexity. On top of that, it seems to me this is more likely to happen to VCPUs that aren't doing very much work and therefore would not suffer a performance loss from migrating physical CPU on occasion.

Are you seeing a lot of these migrations?

I added this small fix to the scheduler to fix this behavior. And

with

it

I see the stability of Xen improved. Win2003 boot was crashing with unhandled MMIO error on xen64 earlier with credit scheduler. I am not seeing that crash with this small fix anymore. It is quiet

possible

that

there are more bugs I need to catch for HVM domains in the credit scheduler. And I would like to know your thoughts for this change.

I don't agree with this change.

When a VCPU is the only member of a CPU's runq, it's still waiting for a _running_ VCPU to yield or block. We should absolutely be picking up such a VCPU to run elsewhere on an idle CPU. Else, you'd end up with two VCPUs time-slicing on a processor while other processors in the system are idle.

Your change effectively turns off migration on systems where the number of active VCPUs is less than 2 multiplied by the number of physical CPUs. I can see why that would hide any bugs in the context migrating paths, but that doesn't make it right. :-)

csched_runq_steal(struct csched_pcpu *spc, int cpu, int pri)

{

struct list_head *iter;

struct csched_vcpu *speer;

struct vcpu *vc;

/* If there are only 1 vcpu in the queue then stealing it from the queue

* is not going not help in load balancing.

*/

if (spc->runq.next->next == &spc->runq)

return NULL;

Thanks & Regards,

Nitin

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Open Source Technology Center, Intel Corp

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