Michael Miles wrote:
> Thank's for the clear up. My question is with Hyperthreading that is if
> each core does double duty so to speak by looking after two threads
> would it not do basically the same work as one core full bore on one thread.
> Is there a speed difference (faster, slower)
Good question. The answer is “it depends, but it’s usually faster”.
Reasons why it can be faster:
* Most modern processors can despatch up to three or four instructions
at a time (IF the front end can identify enough instructions that
logically can be run at the same time), but will have six to ten
execution units to actually run the instructions¹. Therefore, one
thread might be able to make use of execution units the other thread
isn’t using.
* Compared to CPU speed, it takes a seriously long time to get data
from main memory. If one thread is waiting for data to arrive, the
other one can make full use of the processor.
* Most modern CPUs do out-of-order execution, which means they can
often find things to do while waiting for data to come from (L2/L3)
cache. That’s not guaranteed, though, so the other thread might get
more resources to play with.
On the other hand, Atom isn’t out-of-order, and can’t do anything
while it’s waiting for data from Level 2 cache. So the other thread
has full run of the core.
Why it can be slower:
* The cache memory is having to look after two sets of data, not just
one, which means there’ll be a lot more cache misses. The worst case
example would be something like two threads, each of which are
regularly hitting a different 6K of data, on a Pentium 4 with only 8K
Level 1 data cache. Each thread will be constantly replacing the
other’s data, meaning each thread is continually having to wait for
data from Level 2 cache.
This effect was especially noticeable on Pentium 4-based CPUs: a lot of
high-end benchmarks would be run with SMT turned off.
Hope this helps,
James.
¹ The instruction units are specialised: if a thread is 100% integer,
the FPU units won’t be of any use to it.
--
E-mail: james@ | “The duke had a mind that ticked like a clock and, like a
aprilcottage.co.uk | clock, it regularly went cuckoo.”
| -- Terry Pratchett, Wyrd Sisters
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