Windows 7: Hyperthreading quad core system

I recently purchased a system to last me through retirement, so although I'm a light user, I chose a fairly respectable i7 quad core with 16Gb RAM. Plenty of future-proofing there.

Process Explorer shows *eight* CPUs, so presumably the people who configured/supplied this system enabled hyperthreading (or it defaulted to ON). (Belarc Advisor confirms this)



In the past I've seen warnings that hyperthreading can reduce performance as it causes cache contention. The wisdom was "don't use it if you have sufficient CPU". Well, I've rarely seen more than about 5% CPU usage, so I seem to have an abundance.

What is the current consensus on hyperthreading?

Leave it enabled, why buy a hyperthreaded processor to disable ht?

If you manage to disable it, out of the 8 cores only one will work, effectively converting your i7 into a Pentium 4. Not a good choice in my opinion. In fact it will greatly improve performance, as part of the i7 power comes from the multicore architecture. Never heard of anyone saying that it actually reduces performance.

Even if you don't actually use it to 100%, programs will spread over all cores to speed up themselves, by using parallel computing to finish tasks earlier than doing everything in a single core.

I'm quoting from several years back, near the start of hyperthreading, and before even dual core systems were plentiful.
The performance hit came from having two threads running in a single core, with just one, shared, on-board memory cache.
When a single application runs on a single processor, the on-board cache is efficient, as applications localise their reference patterns.
When two different applications hyperthread on a single core, the cache required is much greater to avoid too many cache misses; the two processes "steal" cache space from each other, and both run significantly slower than they would have if run alone.

Newer processors have probably eliminated this, if only by having larger onboard cache space.

However, you may have solved a problem I had before I retired. We were running Ubuntu on an IBM server with four discrete Xeon cpu's. However, the Ubunto boot always failed if we told it to run in multiprocessor mode; perhaps it sees only one processor if hyperthreading is off? Unlikely, I'd have thought since the four processors were in separate physical modules, so it's unlikely that the CPU count would fall to one, but who knows? It's worth testing. Thanks!

Yeah, that was P4 hyperthreading where you only had one CPU to start with.

meh, I can say that they perfected the technology at least for Atoms, when there were still netbooks you could easily notice the difference between a single-core Atom and one with hyperthreading. (hint: the latter was better)