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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. "

9 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.

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    1. Re:Sponsored by VMWare.. what do you expect? by cp.tar · · Score: 3, Insightful

      Even so, they may be at least partially right.

      Besides, if a hybrid approach is necessary, VMWare will need to adjust as well. Or am I missing something?

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    2. Re:Sponsored by VMWare.. what do you expect? by zerogeewhiz · · Score: 3, Insightful

      Haven't read it, but I wonder if they were using VT/Pacifica chipsets or no...

      It's like Apple's claim that their Intel jobbies are 5x faster - a bit silly and very, very specific...

      And yes, VMWare are hardly likely to mention that Xen-style virtualisation is going to be better now, are they?

    3. 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. 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.

  3. Re:Bias? by RegularFry · · Score: 4, Insightful

    Insisting on third-party verification of results is hardly damning either of them... It's just scientific. You (and everyone else) are absolutely right to be sceptical, and not just because VMware have a vested interest in this case. They might just be wrong. Or not.

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  4. wrong by m874t232 · · Score: 3, Insightful

    Hardware virtualization may be slower right now, but both the hardware and the software supporting it are new. Give it a few iterations and it will be equal to software virtualization.

    It may or may not be faster eventually, but that doesn't matter. What matters is that small changes in the hardware make it possible to stop having to depend on costly, proprietary, and complex software--like that sold by VMware.

  5. Use Paravirtualization by graf0z · · Score: 3, Insightful
    Paravirtualization (running hypervisor-aware guest kernels, eg patched linux on xen) is faster than both, binary translation and "full" virtualization. And you don't need CPUs with VT extension.

    g

  6. I think that's a little innacurate by Sycraft-fu · · Score: 3, Insightful

    It's not that people don't look to old mainframe solutions for things, they do, it's that often what was feasable on those wasn't on normal hardware, until receantly. There was no reason for chip makers to waste silicon on virtualization hardware on desktops until fairly receantly, there just wasn't a big desktop virtualization market. Computers are finally powerful to the point that it's worth doing.

    It's no supprise that large, extremely expensive computers get technology before home computers do. You give me $20 million to build something with, I can make it do a lot. You give me $2000, it's going to have to be scaled way back, even with economies of scale.

    You see the same thing with 3D graphics. Most, perhaps even all, the features that come to 3D cards were done on high end visualizaiton systems first. It's not that the 3D companies didn't think of them, it's that they couldn't do it. The orignal Voodoo card wasn't amazing in that it did 3D, it was much more limited than other thigns on the market. It was amazing in that it did it at a price you could afford for a home system. 3dfx would have loved to have a hardware T&L engine, AA features, procedural textures, etc, there just wasn't the silicon budget for it. It's only with more developments that this kind of thing has become feasable.

    So I really doubt Intel didn't do something like VT because they thought IBM was wrong on the 360, I think rather they didn't do it because it wasn't feasable or marketable on desktop chips.