On 8 Aug 2009, Per Jessen told this:
> Nix wrote:
>> On 1 Aug 2009, Linda Walsh stated:
>>
>>> Per Jessen wrote:
>>>> Not sure about that - AFAICT, it's exactly the same technology. (I
>>>> haven't done in exhaustive tests though).
>>>
>>> Supposedly 'Very' different (I hope)...
>> Oh yes. I have a P4 here (2GHz Northwood), and two Nehalems (one 2.6GHz
>> Core i7 with 12Gb RAM and a 2.26GHz L5520 with 24Gb, hello overkill).
>> Compared to the P4s, the Nehalems are *searingly* fast: the performance
>> difference is far higher than I was expecting, and much higher than the
>> clockspeed different would imply.
>
> But how about the core subject here - the hyperthreading? Have you noticed
> anything very different wrt that? I haven't, but it will
> certainly depend on your workload.
Yes. My primary benchmark was a parallel profiledbootstrap of GCC 4.4
sans Java ('cos that requires lots of other infrastructure I didn't have
present at the time): this tests parallel cache-bashing CPU-bashing
stuff, and also tests out the disk subsystem hard because every
compilation launched during stage 3 writes out several dozen megabytes
of profiling state.
Without hyperthreading, 'make -j 6' was the fastest available option,
taking 1392s to do a full bootstrap: turn turbo mode off and that rises
to 1491s. With hyperthreading, 'make -j 11' was fastest, and took 1094s
without turbo mode, 982s with it. Again, not an earthshaking speedup,
but worth it.
By comparison, that P4? It takes *five hours* to do the same job.
982s versus perhaps 18000s... nearly twenty times faster.
