Date   : Fri, 31 Jul 2009 20:45:29 +0100
From   : zeem.uk@... (Alex Taylor)
Subject: Units of memory, Was: Master Ethernet upgrade

2009/7/31 Rob <robert@...>:

> Hmm... all I can say is that with HT on, I could see certain apps
> taking 100% of one of the two "cpus" in task manager, 50% usage
> overall

Ah... it might look like that in the task manager, but I don't think
that's the way it works.

It's a side-effect of the fact that Windows thinks it's got two
processors when in reality it's just one with some enhancements.
Something that shows up as 100% on only one of the CPU graphs will
still be running at full speed, only without any benefit that might
otherwise be brought about by hyperthreading. Sometimes stuff will
just run at the same speed as if HT wasn't there (maybe a little bit
better because operating system threads can be running
simultaneously), sometimes on the P4 there's an actual performance hit
caused by the way it works (see below).

2009/7/31 Andrew Livens <Andy@...>:
> I think that's more down to the way the OS is managing the application
> than the processor architecture.

According to Wikipedia:

"...some programs actually slow down slightly when Hyper Threading
Technology is turned on. This is due to the replay system of the
Pentium 4 tying up valuable execution resources, thereby starving the
other thread. (The Pentium 4 Prescott core gained a replay queue,
which reduces execution time needed for the replay system, but this is
not enough to completely overcome the performance hit.)"

Hyperthreading has been reintroduced on the Atom and Core i7 lines,
which apparently do not suffer the same performance hit that the P4
does, due to the different architecture.

-- 
Alex Taylor