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Old 05-10-2011, 10:28 AM
Adam Carter
 
Default Hyperthreading

I haven't been able to find clear info on Hyperthreading, but from what I can tell it appears that with Hyperthreading On;
1. per core performance is slightly reduced
2. you can run two threads per core, but there is some contention between threads


So, generally, if you have less busy threads than cores, you should leave it off and if you have more busy threads than cores you should turn it on.

Does that sound right?

I assume that newer Nehalem/Core i7 HT (otherwise known as simultaneous multi-threading, SMT) just has less contention between threads than the older P4 HT, but the busy threads vs core principle remains.
 
Old 05-10-2011, 12:11 PM
Joost Roeleveld
 
Default Hyperthreading

On Tuesday 10 May 2011 12:28:01 Adam Carter wrote:
> I haven't been able to find clear info on Hyperthreading, but from what I
> can tell it appears that with Hyperthreading On;
> 1. per core performance is slightly reduced

Not in all circumstances...

> 2. you can run two threads per core, but there is some contention between
> threads

Not in all circumstances...

> So, generally, if you have less busy threads than cores, you should leave
> it off and if you have more busy threads than cores you should turn it on.
>
> Does that sound right?

Nope

HT is based on the theory that not all threads are the same. That means that
certain parts of a core can be kept busy with a completely different task.
If the system is used for lots of different things simultaneously, then HT can
lead to better performance.

However, if the system is doing a lot of identical calculations, then
performance will actually be less as the CPU is trying to find tasks that can
use unused parts. These are, in this case, extremely rare as the vast majority
of CPU-tasks are identical.

> I assume that newer Nehalem/Core i7 HT (otherwise known as simultaneous
> multi-threading, SMT) just has less contention between threads than the
> older P4 HT, but the busy threads vs core principle remains.

HT is still based on the same theory as it was when it was first introduced.
The algorithms are probably improved, but the same problem will occur.

In general, for a desktop or server that is doing a lot of different things, HT
is likely to improve performance.
If the server is dedicated to a single service, there is a distinct chance HT
will lead to decreased performance.

--
Joost
 
Old 05-10-2011, 11:53 PM
Adam Carter
 
Default Hyperthreading

In general, for a desktop or server that is doing a lot of different things, HT

is likely to improve performance.

If the server is dedicated to a single service, there is a distinct chance HT

will lead to decreased performance.


Thanks Joost! That certainly helps.
 

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