Re: [Xen-devel] RE: The caculation of the credit in credit_scheduler

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

On 11/10/10 06:55, Jiang, Yunhong wrote:
> > -----Original Message-----
> > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> > [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Juergen Gross
> > Sent: Wednesday, November 10, 2010 1:46 PM
> > To: Jiang, Yunhong
> > Cc: George Dunlap; xen-devel@xxxxxxxxxxxxxxxxxxx; Dong, Eddie; Zhang, Xiantao
> > Subject: Re: [Xen-devel] RE: The caculation of the credit in credit_scheduler
> >
> > On 11/10/10 03:39, Jiang, Yunhong wrote:
> > > > -----Original Message-----
> > > > From: George Dunlap [mailto:George.Dunlap@xxxxxxxxxxxxx]
> > > > Sent: Tuesday, November 09, 2010 10:27 PM
> > > > To: Jiang, Yunhong
> > > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Dong, Eddie; Zhang, Xiantao
> > > > Subject: Re: The caculation of the credit in credit_scheduler
> > > >
> > > > On 05/11/10 07:06, Jiang, Yunhong wrote:
> > > > > The reason is how the credit is caculated. Although the 3 HVM domains is
> > pinned
> > > > to 2 PCPU and share the 2 CPUs, they will all get 2* 300 credit when credit
> > account.
> > > > That means the I/O intensive HVM domain will never be under credit, thus it will
> > > > preempt the CPU intensive whenever it is boost (i.e. after I/O access to QEMU),
> > and
> > > > it is set to be TS_UNDER only at the tick time, and then, boost again.
> > > >
> > > > I suspect that the real reason you're having trouble is that pinning and
> > > > the credit mechanism don't work very well together.  Instead of pinning,
> > > > have you tried using the cpupools interface to make a 2-cpu pool to put
> > > > the VMs into?  That should allow the credit to be divided appropriately.
> > > I have a quick look in the code, and seems the cpu pool should not help on such
> > situation. The CPU poll only cares about the CPUs a domain can be scheduled, but
> > not about the credit caculation.
> >
> > With cpupools you avoid the pinning. This will result in better credit
> > calculation results.
> My system is doing testing, so I can't do the experiment now, but I'm not sure 
> if the cpupool will help the credit caculation.
>
> > From the code in csched_acct() at "common/sched_credit.c", the credit_fair is 
> > caculated as followed, and the creadt_fair's original value is caculated by sum all 
> > pcpu's credit, without considering the cpu poll.
>          credit_fair = ( ( credit_total
>                            * sdom->weight
>                            * sdom->active_vcpu_count )
>                          + (weight_total - 1)
>                        ) / weight_total;
>
> Or did I missed anything?
The scheduler sees only the pcpus and domains in the pool, as it is cpupool
specific.
BTW: the credit scheduler's problem with cpu pinning was the main reason for
introducing cpupools.


Juergen

--
Juergen Gross                 Principal Developer Operating Systems
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