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Hardware Virtualization Slower Than Software?

Posted by ryuzaki0 on from the the-jury-is-still-out dept.

Jim Buzbee writes "Those you keeping up with the latest virtualization techniques being offered by both Intel and AMD will be interested in a new white paper by VMWare that comes to the surprising conclusion that hardware-assisted x86 virtualization oftentimes fails to outperform software-assisted virtualization. My reading of the paper says that this counterintuitive result is often due to the fact that hardware-assisted virtualization relies on expensive traps to catch privileged instructions while software-assisted virtualization uses inexpensive software substitutions. One example given is compilation of a Linux kernel under a virtualized Linux OS. Native wall-clock time: 265 seconds. Software-assisted virtualization: 393 seconds. Hardware-assisted virtualization: 484 seconds. Ouch. It sounds to me like a hybrid approach may be the best answer to the virtualization problem. "

4 of 197 comments (clear)

  1. Sponsored by VMWare.. what do you expect? by thegrassyknowl · · Score: 5, Insightful

    See title... VMWare make software virtualisation products. Of course they're going to try and find that software methods are better.

    --
    I drink to make other people interesting!
    1. Re:Sponsored by VMWare.. what do you expect? by XMLsucks · · Score: 5, Insightful

      VMware sells both hardware-accelerated and software virtualization products. They implemented full support for VT (how else would they benchmark it? Plus they were the first to support VT). If you run VMware on 64-bit Windows, then you use VMware's VT product. But because VMware's original software method is faster than the VT method on 32-bit, they continue to use the software approach.

      VMware's paper is a typical research paper, published at a peer-reviewed conference. This means that they have used the scientific method. The chances are 99.9999% that you will easily reproduce their results, even if changing the benchmarks.

      I, on the other hand, am smart enough to see that they are stating the obvious. If you read the Intel VT spec, you'll see that Intel does nothing for page table virtualization, nor anything for device virtualization. Both are extremely expensive, and besides sti/cli, are the prime candidates for hardware assists. Intel will likely solve this performance issue in future revs, but right now, VT isn't fast enough.

      Hmmm, virtualisation? Do you happen to work on Xen?

    2. Re:Sponsored by VMWare.. what do you expect? by Anonymous Coward · · Score: 5, Informative
      See title... VMWare make software virtualisation products. Of course they're going to try and find that software methods are better.

      Disclaimer: I work for VMware.

      1. VMware already supports VT, but it's not enabled by default because for normal workloads it's slower. If VT really were faster, do you really think we'd be choosing to use a slower approach and making customers unhappy?
      2. Even Intel admits the first generation of VT hardware wasn't so great and now claims that they were aiming for correctness instead of performance:
  2. The correct conclusion is more limited by njdj · · Score: 5, Insightful

    The correct conclusion is not that virtualization is better done entirely in software, but that current hardware assists to virtualization are badly designed. As the complete article points out, the hardware features need to be designed to support the software - not in isolation.

    It reminds me of an influential paper in the RISC/CISC debate, about 20 years ago. Somebody wrote a C compiler for the VAX that output only a RISC-like subset of the VAX instruction set. The generated code ran faster than the output of the standard VAX compiler, which used the whole (CISC) VAX instruction set. The naive conclusion was that complex instructions are useless. The correct conclusion was that the original VAX compiler was a pile of manure.

    The similarity of the two situations is that it's a mistake to draw a general conclusion about the relative merits of two technologies, based on just one example of each. You have to consider the quality of the implementations - how the technology has been used.