Re: Hyperthreading performance oddities

[Willy Tarreau <w@1wt.eu>]

Hi Andi,

On Fri, Mar 07, 2008 at 08:20:32PM +0100, Andi Kleen wrote:
> Chris Snook <csnook@redhat.com> writes:
> > 
> > Turning on hyperthreading effectively halves the amount of cache
> > available for each logical CPU when both are doing work, which can do
> > more harm than good.
> 
> When the two cores are in the same address space (as in being two
> threads of the same process) L1 cache will be shared on P4. I think
> for the other cases the cache management is also a little more
> sophisticated than a simple split, depending on which HT generation
> you're talking about (Intel had at least 4 generations out, each with
> improvements over the earlier ones)

Oh that's quite interesting to know.

> BTW your argument would be in theory true also for multi core with
> shared L2 or L3, but even there the CPUs tend to be more sophisticated.
> e.g. Core2 has a mechanism called "adaptive cache" which allows one
> Core to use significantly more of the L2 in some cases.
>
> >  Number-crunching applications that utilize the
> > cache effectively generally don't benefit from hyperthreading,
> > particularly floating-point-intensive ones.
> 
> That sounds like a far too broad over generalization to me.
> 
> -Andi (who personally always liked HT)

Well, in my experience, except for compiling, HT has always caused
massive slowdowns, especially on network-intensive applications.
Basically, network perf took a 20-30% hit, while compiling took
20-30% boost. But I must admit that I never tried HT on anything
more recent than a P4, maybe things have changed since.

regards,
willy