xeon hyperthreading & GIL

seb bacon seb.bacon at jamkit.com
Mon Feb 10 14:48:55 CET 2003


Aahz wrote:
> In article <1044623847.893400 at ananke.eclipse.net.uk>,
> Seb Bacon  <seb.bacon at jamkit.com> wrote:
> 
>>Let me try again:
>>
>>Currently python's global interpreter lock will often cause SMP system 
>>performance to *decrease* due to the cost of context switching.
> 
> 
> Wrong.  It's not the cost of context switching that causes the
> performance decrease (or at least no more than for an equivalent C
> program).  It's the inability to schedule multiple simultaneous Python
> threads.  In order to get a performance boost (and this applies to
> single-CPU machines just as much as SMP), you need to call out to an
> extension that releases the GIL.

Stating the completely obvious, but there is also  an inability to 
schedule multiple simlutaneous threads on a single CPU.

Therefore, the fact that you can't schedule them on SMP does not explain 
why SMP performs worse than single-CPU in many python applications.

I understood that SMP is slower because context switching between 
processors is more expensive than switching contexts on a single 
processor.  You can certainly improve SMP performance by increasing the 
interval check on the lock, which kind of implies that my understanding 
is correct?

The relevant part of your response to my original question is "at least 
no more than for an equivalent C program".

Given an equivalent C program, and no rewriting of python code outside 
the GIL, how will the performance of a Xeon compare to that of an 
equivalent SMP system?





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