Re: [Xen-devel] credit vs sedf scheduler

["Liang Yang" <multisyncfe991@xxxxxxxxxxx>]

Hi Emmanuel,

Thanks a lot for your reply and this is clear now.

When you are talking about CPU scheduler, another question just came into 
mind: Xen manual mentioned Xen used kind of depth-first algorithm to 
determine the CPU sequence on a platform which has two processor and each 
processor has two cores and each core has two hyper-threads. That means, 
CPU0 will be the first thread in the first core in the first processor, CPU1 
will be the second thread in the first core in the first processor, etc. I 
have a dual-processor/dual-core platform and it seems this is true by 
disabling/enabling multi-core in BIOS.
However, I was told the CPU sequence is not numbered using this depth-first 
scheme on the latest Intel quad-core platform. I don't understand why Xen 
need to change this.
Do you have any idea how to verify the CPU sequence number on a 
dual-processor/quad-core/hyper-threaded platforms?
Best regards,

Liang

----- Original Message ----- 
From: "Emmanuel Ackaouy" <ack@xxxxxxxxxxxxx>
To: "Liang Yang" <yangliang_mr@xxxxxxxxxxx>
Cc: "xen-devel" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Sent: Thursday, November 16, 2006 10:22 AM
Subject: Re: [Xen-devel] credit vs sedf scheduler


> On Thu, Nov 16, 2006 at 10:08:40AM -0700, Liang Yang wrote:
> > Hi Emmanuel,
> >
> > Do you mean, if the number of VCPUs is equal to the number of physcial
> > CPUs, Xen will still call credit scheduler to allocate CPU resources
> > amongst V CPUs even though each guest domain is assigned a fixed VCPU/CPU
> > using xm vcpu-pin?
> What I mean is that:
>
> 1- The clock interrupt will still run every 10ms and this will
> run some scheduler accounting code.
> 2- After a VCPU completes its time slice, Xen will call into
> the scheduler code to see if anything else needs to run. Of
> course, in this case, there will be nothing else to run so the
> VCPU will continue running.
>
> Both of these should be absolutely negligeable.
>
> Are you seeing overhead which is not negligeable? If so, this
> would be a bug and should be fixed.
>
> > The purpose I assign each domain a CPU and fix the mapping of VCPU to CPU
> > is I want each guest domain get equal CPU resources at any time. I don't
> > want to credit scheduler get involved.
> If your goal is to have each domain get equal CPU at any given
> time, then you don't need to do anything. The scheduler will
> automatically spread your VCPUs across the 4 physical CPUs and
> they will each get an equal amount of physical CPU resources.
>
> Also keep in mind that if a 5th domain (dom0 for example) needs
> CPU resources as well, the credit scheduler will do this fairly
> and also with negligeable costs whereas your scheme to pin all
> VCPUs on their own physical CPU will not.
>
> Cheers,
> Emmanuel.
>
> > Regards,
> >
> > Liang
> > ----- Original Message ----- 
> > From: "Emmanuel Ackaouy" <ack@xxxxxxxxxxxxx>
> > To: "Liang Yang" <multisyncfe991@xxxxxxxxxxx>
> > Cc: "xen-devel" <xen-devel@xxxxxxxxxxxxxxxxxxx>
> > Sent: Thursday, November 16, 2006 9:37 AM
> > Subject: Re: [Xen-devel] credit vs sedf scheduler
> >
> >
> > >On Thu, Nov 16, 2006 at 09:29:30AM -0700, Liang Yang wrote:
> > >>   Hi,
> > >>
> > >>   Can I used vcpu-pin to avoid CPU scheduler overhead? Suppose I have
> > >>four
> > >>   CPUs, if I used xm vcpu-pin to fix the mapping of each physcial CPU 
> > >> to
> > >>   virtual CPU. So credit scheduler will not be needed.
> > >>
> > >>   Liang
> > >
> > >Pinning all your VCPUs will not stop Xen from executing various
> > >scheduler code paths.
> > >
> > >Why are you worried about the scheduler overhead? It should be
> > >negligeable.
> > >
> > >Emmanuel.
> > >

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