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Re: [Xen-users] DomU slower with smp?

To: Nick Anderson <nick@xxxxxxxxxxxx>
Subject: Re: [Xen-users] DomU slower with smp?
From: Luke S Crawford <lsc@xxxxxxxxx>
Date: 15 May 2008 21:54:34 -0400
Cc: Janusz Ulanowski <janul666@xxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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Nick Anderson <nick@xxxxxxxxxxxx> writes:
> Its a dual socket quad core amd. So I have 8 cores. cpus for dom0 is
> set to 0 and according to xend-config.sxp that will give dom0 access
> to all cpus. I havent adjusted any weighting so im not sure what would
> be stepping on it. The only thing running during the test was that one
> domU. and the domU was idle when running the test on dom0.

here is what I'm talking about:
[root@coloma lsc]# xm vcpu-list 0
Name                              ID VCPUs   CPU State   Time(s) CPU Affinity
Domain-0                           0     0     0   -b-   33162.5 any cpu
Domain-0                           0     1     1   r--    6688.2 any cpu

if you notice, Dom0 has access to 2 CPUs, but most of the work is done on
the first CPU.  I/O, generally speaking, is not an operation that
threads well.  

[root@coloma lsc]# xm vcpu-list oggfrog
Name                              ID VCPUs   CPU State   Time(s) CPU Affinity
oggfrog                          124     0     0   -b-    2160.2 any cpu

one of the DomUs, oggfrog, is also on CPU 0... however, I've set the weights:

[root@coloma lsc]# xm sched-credit -d 0
{'cap': 0, 'weight': 4096}
[root@coloma lsc]# xm sched-credit -d oggfrog
{'cap': 0, 'weight': 256}

before I set the weights, IO was pretty bad.  This way is acceptable,
but it would probably be significantly faster if I gave a whole CPU to
the Dom0 (but that box only has 2 cores, so I'm sharing-  this gives my 
DomUs better CPU but worse IO performance.)  

the way to see if this is your problem is to execute the xm vcpu-list commands
while reproducing the problem in the SMP and Non-SMP DomU. 

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