Re: Virtualization Performance: Intel vs. AMD

Thomas Fjellstrom
2009-11-15T23:03:35Z

On Sun November 15 2009, Gordan Bobic wrote:
> Thomas Fjellstrom wrote:
> > On Sun November 15 2009, Neil Aggarwal wrote:
> >>> The Core i7 has hyperthreading, so you see 8 logical CPUs.
> >>
> >> Are you saying the AMD processors do not have hyperthreading?
> >
> > Course not. Hyperthreading is dubious at best.
>
> That's a rather questionable answer to a rather broad issue. SMT is
> useful, especially on processors with deep pipelines (think Pentium 4 -
> and in general, deeper pipelines tend to be required for higher clock
> speeds), because it reduces the number of context switches. Context
> switches are certainly one of the most expensive operations if not the
> most expensive operation you can do on a processor, and typically
> requires flushing the pipelines. Double the number of hardware threads,
> and you halve the number of context switches.

Hardware context switches aren't free either. And while it really has
nothing to do with this discussion, the P4 arch was far from perfect (many
would say, far from GOOD).

> This typically isn't useful if your CPU is processing one
> single-threaded application 99% of the time, but on a loaded server it
> can make a significant difference to throughput.

I'll buy that. Though you'll have to agree that the initial Hyperthread
implementation in intel cpus was really bad. I hear good things about the
latest version though.

But hey, if you can stick more cores in, or do what AMD is doing with its
upcoming line, why not do that? Hyperthreading seems like more of a gimmick
than anything. What seems to help the most with the new Intel arch is the
auto overclocking when some cores are idle. Far more of a performance
improvement than Hyperthreading will ever be it seems.

But maybe that's just me.

> Gordan
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--
Thomas Fjellstrom
tfjellstrom@shaw.ca