How does Windows exploit hyperthreading?
It depends which version of Windows you’re asking about.
For Windows 95, Windows 98, and Windows Me,
the answer is simple: Not at all.
These are not multiprocessor operating systems.
For Windows NT and Windows 2000, the answer is
“It doesn’t even know.”
These operating systems are not hyperthreading-aware
because they were written before hyperthreading was invented.
If you enable hyperthreading, then each of your CPUs looks
like two separate CPUs to these operating systems.
(And will get charged as two separate CPUs for licensing
Since the scheduler doesn’t realize the connection between
the virtual CPUs, it can end up doing a worse job than
if you had never enabled hyperthreading to begin with.
Consider a dual-hyperthreaded-processor machine.
There are two physical processors A and B, each with
two virtual hyperthreaded processors, call them A1, A2,
B1, and B2.
Suppose you have two CPU-intensive tasks.
As far as the Windows NT
and Windows 2000 schedulers are concerned, all four
processors are equivalent, so it figure it doesn’t matter which two
it uses. And if you’re unlucky, it’ll pick
A1 and A2, forcing one physical processor to shoulder two
heavy loads (each of which will probably run at something
between half-speed and three-quarter speed),
leaving physical processor B idle;
completely unaware that it could have done a better job
by putting one on A1 and the other on B1.
Windows XP and Windows Server 2003 are hyperthreading-aware.
When faced with the above scenario, those schedulers will know
that it is better to put one task on one of the A’s and the other
on one of the B’s.
Note that even with a hyperthreading-aware processor,
you can concoct pathological scenarios where hyperthreading ends
up a net loss. (For example, if you have four tasks, two of which
rely heavily on L2 cache and two of which don’t, you’d be better
off putting each of the L2-intensive tasks on separate processors,
since the L2 cache is shared by the two virtual processors.
Putting them both on the same processor would result in a lot of L2-cache
misses as the two tasks fight over L2 cache slots.)
When you go to the expensive end of the scale (the Datacenter Servers,
the Enterprise Servers), things get tricky again.
I refer still-interested parties to the
Windows Support for Hyper-Threading Technology white paper.
Update 06/2007: The white paper
appears to have moved.
Update 10/2016: The white paper