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[Xen-devel] Did xen do some cache prefetch? -- Sisu

Hi, all:

I am trying to studying the cache effect under Xen.

I found under virtualization, the cache latency is smaller than under a non-virt OS using same machine.

I am using similar technogies as in post, Figure 1.

My host domain is Cent OS 6.2, running Linux 3.4.35,
Xen version is 4.2.0
Guest OS is Ubuntu 12.04, Kernel 3.2.0.

My CPU is Intel i7, x980, it has 6 cores, runs constantly at 3.33GHz (disabled hyperthread). Also disabled frequency scaling.

with 32KB L1, 256KB L2 on each core, and a shared 12 MB L3 cache.

I am measuring the cache latency, the workload is readling data from an array.

X axis is the array size, in log scale. y axis is the average cycles per access.

Two lines are shown:
The solid line is an experiments done using non-virt OS. I pinned the task to a specific core to prevent migration;
The dashed line is running the same experiment, but within a guest OS(configured with one VCPU, pinned to one core).

You can see both lines shows three jumps at 32KB, 256KB, and around 12MB, which is the size of L1, L2, and L3.

The strange thing is the time using virtualization is smaller than non-virt.

I am guessing Xen did some cache prefetch about this?

Thanks very much!


Sisu Xi, PhD Candidate

Department of Computer Science and Engineering
Campus Box 1045
Washington University in St. Louis
One Brookings Drive
St. Louis, MO 63130

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