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xen-devel
Re: [Xen-devel] multi-processor odd behavior?
Hi John,
In addition to the default round robin allocation, we have special
behaviour for HT. This behaviour is actually by design; from
xen/common/dom0_ops.c:
----- 200 * If we're on a HT system, we only use the first HT for dom0,
other 201 * domains will all share the second HT of each CPU. Since dom0 is
on 202 * CPU 0, we favour high numbered CPUs in the event of a tie. 203 */
204 pro = smp_num_siblings - 1; 205 for ( i = pro; i < num_online_cpus(); i
+= smp_num_siblings ) 206 if ( cnt[i] <= cnt[pro] ) 207 pro = i; -----
This is because you get IO performance gains (particularly for demanding
loads, like small packets on a GigE network) by having dom0 run *truly*
concurrently with the domUs it is servicing. This is because it saves
context switching overhead.
The domUs are therefore running on the second HT. The CPU's HT
implementation should ensure that if dom0 is idle, the second HT will get
all the raw horsepower of the processor.
If you really want to run domUs on both threads, you can use the pincpu
command to manually shuffle them around.
Cheers,
Mark
On Nov 18 2005, John L Griffin wrote:
Hey folks,
I'm using a recent (dated last weekend) xen-unstable build on a machine
with two processors. (Well, one processor with hyperthreading turned on.)
I've created two guest domains. Inside each domain, I have a simple
application ("eat") that's a simple "for(;;);" loop, to consume as much
CPU as possible.
I'm seeing odd behavior:
1. When I run "eat" inside one guest, "xm top" reports for CPU(%):
Domain-0: 0.5
guest-1: 99.6
guest-2: 0.0
2. When I run "eat" inside both guests:
Domain-0: 0.5
guest-1: 49.9
guest-2: 49.9
[This isn't what I expect, which is for both domains to run near 100%.]
3. When I run "eat" inside both guests *and* in Domain-0:
Domain-0: 97.6
guest-1: 49.9
guest-2: 49.9
This seems to indicate that one CPU is being reserved entirely for dom0,
even when dom0 isn't using it to capacity? (That doesn't seem like the
intended behavior.) Do I have a configuration option set wrong? Or is
this a known issue? Or, is it fixed in the latest builds?
"xm top" shows "CPUs: 2 @ 2992MHz", and also indicates that Domain-0 has 2
VCPUS while each of the guests has 1 VCPU.
Thanks,
JLG
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