Re: [Xen-devel] [PATCH] vcpu pin weight considerstion TAKE3

[Atsushi SAKAI <sakaia@xxxxxxxxxxxxxx>]

Hi, Emmanuel

 Thank you for commenting and suggestion of my patch.
I comment in line.

If pcpu group(or partitioning) API is acceptable, that is simple way to 
solve.



Emmanuel Ackaouy <ackaouy@xxxxxxxxx> wrote:

> Hi.
> 
> Before I discuss the approach again, let me first comment on your
> patch:
> 
> I don't understand why you need to keep track of the subset of
> active VCPUs which are online. An active VCPU is one which
> has recently consumed credits. I don't understand the point of
> ignoring those which are currently blocked when you look. Can
> you explain?

This is not blocked one, _VPF_down one.
Blocked vcpu is counted in this code(online_vcpu_count).
So This is for online vcpu reduced by  xm vcpu-sets.

> 
> 
> Now to step back a little and look at the approach itself:
> 
> Weights are useful to proportionally allocate credits to domains
> which are competing for the _same_(!!!) CPU resources. When
> you restrict where a VCPU can run, you break this assumption.

My case is for pcpu.credit(vcpu.credit sum) > 30msec.
To appropriate weighting, I reduced each vcpu credit with same ratio.

> 
> You are attempting to divide the total weight among different
> "zones" by assigning a restricted VCPU's share of the total
> weight to the physical CPU where it last ran. This will work
> as long as your distinct "zones" do not overlap. When they
> do overlap, this will not work.

Yes, you are right.
I am considering vcpu-pinned domain view.
(c.f.My configure is domU1 and domU2 is pinned to pcpu0-1 only 
on 4pcpu machine.)
>From not pinned domain view, skipped weight should give to 
to the non-pinned vcpu.
Current patch gives same CPU resources to the non-pinned vcpu
after comsuming credit.
(since credit is shorten by vcpu-pinned pcpu.credit >30msec)
It should be treat vcpu credit should give based on vcpu weight.
I will fix it.
Thank you for your suggestion.


> To be specific, in summing_vcpupin_weight(), you assign a
> restricted VCPU's share of the total weight to the CPU it last
> ran on. This assumes that all other competing VCPUs will
> also sum over that physical CPU. This isn't true when distinct
> CPU affinity masks ("zones") can overlap.

Yes,
For this correct handling, previous TAKE2 patch calculates each pcpu weight 
based on vcpu-pin information.
In this TAKE3 patch, some vcpu-pin overlap case is problematic.
(not all of the cases is problematic.)
In this mean, this patch does not purposes partitioning at first.


> 
> When all distinct CPU affinity masks do not overlap with each
> other, the host system has essentially been partitioned.
> 
> Partitioning is an interesting subset of the general problem
> because:
> - It is easily defined
> - it has a simple and elegant solution.
> 
> Your solution only works when the system has been properly
> partitioned. This is fine. Actually, the problem of handling
> overlapping CPU affinity masks is not a tractable one.
> 
For this properly handing, TAKE2 patch have vcpu-pin weight handling.

Anyway CPU grouping is another solution to solve.
If permittable, I am willing re-write it.


> What I argue is that there is a cleaner approach to deal with
> the partitioning of the host: For the most part, you only have
> to allow multiple csched_privates structures to co-exist. Each
> such group will have a master CPU doing its accounting just
> like today. The accounting code will be left mostly untouched:
> You probably just need to AND your group's assigned CPUs
> to the online mask here and there.
> 
> Keeping the accounting work the same and distributing it
> across CPUs is more desirable than adding complexity while
> still keeping it on a single CPU.
> 
> Personally I think Xen would also benefit from having a clean
> interface for managing CPU partitions but this isn't strictly
> necessary.
> 
> Unfortunately, having left XenSource some months ago, I
> have virtually no time to spend on Xen. I would love to help
> review patches that add partitioning support the way I have
> described: By allowing the co-existence of independent
> scheduling "zones", each represented by its distinct version
> of the current csched_private structure. I appreciate the work
> you are doing trying to tackle this problem but I think your
> patches are the wrong way forward. Given that, I'm not going
> to be able to spend time looking at further re-spins of this
> patch.
> 
> Cheers,
> Emmanuel.
> 
> On Jul 10, 2007, at 10:47, Atsushi SAKAI wrote:
> 
> > Hi, Keir and Emmanuel
> >
> > This patch intends to correct the weight treatment for vcpu-pin.
> > Would you give me a comment on this?
> >
> >  sched_credit.c |  115 
> > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> >  1 files changed, 114 insertions(+), 1 deletion(-)
> >
> >
> > Current algorithm is
> > 1)calculate pcpu_weight based on all active vcpu.
> >   vcpu_weight = sdom->weight/sdom->online_vcpu_count(newly added 
> > variable)
> >   v->processor
> >   (This routine runs when vcpu count is changed, not every 30msec.)
> >
> > 2)calulate vcpupin_factor based on
> >   avarage of 
> > vcpu-pinned-pcpu_weight/(csched_priv.weight/num_online_cpu())
> >
> > 3)consider above factor when credit is added in vcpu-pin case.
> >   at atomic_add(credit_fair, &svc->credit);
> >
> > Signed-off-by: Atsushi SAKAI <sakaia@xxxxxxxxxxxxxx>
> >
> > Difference from previous patch(+365 lines) is
> >
> > 1)credit_balance consideration factor is omitted (about -150 lines)
> > 2)detailed pcpu_weight calculation is changed (about -100 lines)
> >   (currently uses v->processor instead of vcpu->cpu_affinity)
> >
> > Thanks
> > Atsushi SAKAI
> >
> > <vcpupin0710.patch>_______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> 
> 
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Thanks
Atsushi SAKAI




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