hyper threading and memory usage

[Micah DesJardins]

As I understand it, there's some slight issues with the way
hyperthreading CPUs deal with sharing L1 and L2 cache between the
'independent CPU' threads.  In theory, in a properly optimized SMP
aware program the benefit of avoiding pipeline stalls through
hyperthreading far outweighs any performance loss from having to take
a few more frequent L2 cache hits.  In practice, if you're dealing
with non-threaded or non-SMP apps you may notice a performance hit.  I
know this is something they were working on but I have no idea where
they are at with it currently.

The idea behind Hyperthreading, is to help eliminate pipeline stalls
in the first place.  Personally, I prefer AMD's approach
(Hypertransport) which is to speed up the memory bus to reduce stalls
by making the front side bus 'a lot faster' to access all available
memory.  Both technologies have their merits and to each their own and
all that jazz.

A couple of links:

http://news.zdnet.co.uk/0,39020330,39237341,00.htm

http://softwareforums.intel.com/ids/board/message?board.id=42&message.id=621

Micah

On 5/16/06, Nathan England <nathan at paysonlinux.org> wrote:
>
> I just got a hyperthreaded computer and I'm noticing that it doesn't handle
> memory as well as my other machines.
> I haven't been doing anything different, infact I ghosted my hdd to the new
> machine, so everything is identicle, except I built the kernel for SMP.
> It only has 512MB right now, but so do my other machines that do about the
> same amount of work.
>
> The only difference I notice is the HT'd machine will begin swapping much
> faster than the other machines. Then it gets really slow.
>
> I'm not a big fan of HT to begin with as I'd rather have dual-core or just
> dual cpu, but is this typical? Or is something wrong?
>
> Nathan
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