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[Xen-devel] Odd CPU Scheduling Behavior

  • To: <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "Carb, Brian A" <Brian.Carb@xxxxxxxxxx>
  • Date: Thu, 29 Mar 2007 10:58:09 -0400
  • Delivery-date: Thu, 29 Mar 2007 16:02:36 +0100
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcdyEqoY2lwl6GDzSom5tvvwfsw7sw==
  • Thread-topic: Odd CPU Scheduling Behavior

We're seeing a cpu scheduling behavior in Xen and we're wondering if anyone can explain it.
We're running XEN 3.0.4 on a Unisys ES7000/one with 8 CPUs (4 dual-core sockets) and 32GB memory. XEN is built on SLES10, and the system is booted with dom0_mem=512mb. We have 2 para-virtual machines, each booted with 2 vcpus and 2GB memory, and each running SLES10 and Apache2 with worker multi-processing modules.
The vcpus of dom0, vm1 and vm2 are pinned as follows:
dom0 is relegated to 2 vcpus (xm vcpu-set 0 2) and these are pinned to cpus 0-1
vm1 uses 2 vcpus pinned to cpus 2-3
vm2 uses 2 vcpus pinned to cpus 2-3
The cpus 4 through 7 are left unused.
Our test runs http_load against the Apache2 web servers in the 2 vms. Since Apache2 is using worker multi-processing modules, we expect that each vm will spread its load over the 2 vcpus, and during the test we have verified this using top and sar inside a vm console.
The odd behavior occurs when we monitor cpu usage using xenmon in interactive mode. By pressing "c", we can observe the load on each of the cpus. When we examine cpus 2 and 3 initially, each is used equally by vm1 and vm2. However, shortly after we start our testing, cpu2 runs vm1 exclusively 100% of the time, and cpu3 runs vm2 100% of the time.  When the test completes, CPUs 2 and 3 go back to sharing the load of vm1 and vm2.
Is this the expected behavior?

brian carb
unisys corporation - malvern, pa

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