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Ubuntu 12.10: Linux KVM vs. Xen Virtualization Preview

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  • #16
    Originally posted by tangram View Post
    @ pasik
    The article doesn't state PVonHVM. It states HVM which ends up being a different beast. Also withh HVM you have emulation overhead.
    Xen PVHVM drivers are in upstream Linux kernel, so I believe they used PVHVM. If they didn't, then the benchmark is pointless, because they used VirtIO for KVM guests! (VirtIO is the PV driver for KVM).

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    • #17
      Originally posted by M1kkko View Post
      The conclusion from this article: Run your operating system(s) on bare metal.
      That would be the conclusion if this so called benchmark was run on the same hardware.

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      • #18
        It has already been pointed out that Xen has various options for virtualization. Benchmarks can be very helpful, particularly if they are setup to provide optimum performance. The exact setup must be documented.

        I was surprised to see Xen tested with an Ubuntu HVM guest, and from the discussion here I gather that this guest wasn't even using PVHVM drivers.

        +1 for tangram and pasik and others who pointed that out.

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        • #19
          I had a second look at the benchmark and noticed that Xen was using ext4 partitions. IIRC the Xen wiki clearly states that Xen should be used with LVM for best disk I/O performance. Of course one would have to use the PVoHVM drivers to reach the full potential under HVM, and as already mentioned before, why not use a PV guest in the first place?

          Running a benchmark where one contender is set up in a more or less optimal way (KVM with VirtIO driver) and the other (Xen) using what seems to be a low-performance setup is highly questionable. It hasn't been explained why HVM was chosen over PV (or why not using both options?), nor is it clear whether or not the PVoHVM drivers were used. It looks like most of the tests where Xen under-performs are disk I/O related.

          Since I'm running a Xen system and I don't experience such performance issues, perhaps there is something wrong with the benchmark? For reference, my Windows 7 HVM guest achieves a Windows Experience Index (WEI) of 7.8 (of 7.9) for disk I/O using a SSD and GPLPV drivers. The driver itself helped improve the WEI by 1 index point.

          The way it stands now, the conclusion of the benchmark is simply misleading.

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