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Hardware Virtualization Slower Than Software?
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.
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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!
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.
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.
Reality is the ultimate Rorschach.
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.
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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.
I am 100% in favor of cheap and open solutions. But I don't agree that this will soon be the case for virtualization. VMWare and the few other major vendors do a lot more than software virtualization of a CPU (which is all TFA was talking about). To have a complete virtualization solution, you need to also virtualize the rest of the hardware: storage, graphics, input/output, etc. In particular graphics is a serious issue (attaining hardware acceleration in a virtual environment safely), which from last I heard VMWare were working hard on.
Furthermore, Virtualization complements well with software that can migrate VMs (based on load or failure), and so forth. So, even if hardware CPU virtualization is to be desired - I agree with you on that - that won't suddenly make virtualization as a whole a simple task.
IBM's VM also started as a software product that had to cope with virtualisation problems in the hardware.
Just like what is happening now, they added specific support to the hardware to make VM perform better.
This all happened before the development of today's architectures, but in the early days of microcomputing, IBM had the position that Microsoft has today: they were the big company that had 90% of the market, and in the eyes of the newcomers all they did was by definition the wrong thing. So nobody would bother to look at 360 mainframes, VM and how it was done before designing their own processor.
(this would be similar to telling a Linux geek to look at how certain problems are solved in Windows... it is Windows, it is Microsoft, so it has to be the wrong solution)
I haven't read the results, and I doubt I have the technical knowledge to properly analyze them properly. However if I were to guess as to why this might be the case I'd say it's because they didn't do it right. This is a new and fairly complex technology, I somehow doubt it's easy to get right on the first try.
I am not willing, based on a single datapoint, to make any conclusions. That's tanget to my point anyhow, my point was that doing something in hardware and software are quite different.
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.
Apparently, yes, and by a good margin.
There are several documents and articles out there which point out VT's problems and how Pacifica is quite dramatically better. Here's an excerpt from "AMD Pacifica turns the nested tables", part 3 of an informative series of articles:
This should allow an otherwise identical VMM to do more things in hardware and have lower overhead than VT. AMD appears to have used the added capability wisely, giving them a faster and as far as memory goes, more secure virtualisation platform."
So, it looks like AMD are ahead on hardware virtualization at the moment.
If I read it correctly, this is because Intel's VT actually requires a lot of software intervention, so it's not actually a very strong hardware solution at all.
"The question of whether machines can think is no more interesting than [] whether submarines can swim" - Dijkstra
I designed one of the x86 h/w virtualization offerings. It's obvious that outside of device emulation, the biggest overhead of virtualization is the s/w emulation of what amounts to two levels of address translation (especially hairy in multiprocesor systems due to the brain-dead x86 page table semantics that do not require explicit invalidation). So clearly you want nested-paging support in h/w. However, that support is a little more complex than a few microcode changes to trap selected privileged instructions --- and due to schedule pressures, it didn't make it into the current release. Once that's in, expect h/w virtualization to speed up significantly.
Note that this doesn't make all the other stuff in VT/SVM useless; there are lots of places on the x86 where pure s/w virtualization has to go to great lengths of complexity just to get things correct. As a simple example: there's no way on "old" x86 h/w to save & restore segment descriptors (which you need to do on world switch) --- all you get is the selector, and if the guest O/S has overwritten the in-memory copy, you're out of luck. "Fixable" in s/w (obviously; VMWare does it), but just plain grody. So a major advantage of SVM/VT is that it becomes a lot *easier* to write a VMM (opening up the market to more players; this is starting to show in the Macintosh market) --- eventually, it should become faster, too.
On a separate note, over the next years, expect h/w assistance for dealing the device emulation (and not just from the CPU vendors).