[Xen-users] Unexpected memory bandwidth overhead with Xen

[rohan nigam <locaterohan@xxxxxxxxx>]

Hello Everyone,

While benchmarking a Dell Node (Dual Socket Quad Core AMD Opteron 2354 with 8 GB of memory & CentOS 5.4 installed) , I am getting approx 25-30% overhead with xen after running the STREAM memory bandwidth benchmark using 8 threads with different compilers (gcc, pgi and icc).

These are the results with and without running xen kernel.
Centos:
Kernel Version: 2.6.18-194.17.1.el5
Kernel Version: 2.6.18-194.17.1.el5xen

Function	pgcc	pgcc-xen	Variation (%)
Copy	17108.0186	11827.4346	30.86613432
Scale	16179.1128	11692.2545	27.7324125
Add	16706.097	12212.2696	26.89932544
Triad	17211.552	12936.0666	24.84078949
Function	icc	icc-xen	Variation (%)
Copy	16731.7459	11743.8163	29.81117231
Scale	16026.4231	11560.9261	27.86334151
Add	16656.4325	12077.4699	27.49065624
Triad	16701.5193	12021.5076	28.02147287
Function	gcc	gcc-xen	Variation (%)
Copy	11762.6266	8810.7558	25.09533712
Scale	11499.5329	8611.5352	25.11404355
Add	12399.1248	9388.1446	24.28381235
Triad	12607.9727	9531.4749	24.40120924

I know there are new kernels available but I never heard of such a bad performance for bandwidth numbers for Xen. Does anyone of you know the reason?

Also, HPL on the other hand with the same setup gave a reasonably expected performance overhead for Gflops count. It was approx 3.9-4% overhead with xen.

Does Xen behave like this with memory intensive applications?

Any comments and suggestions will really help.

Thanks,
Rohan

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