python threads on multi-CPU machines

[Thomas Womack]

If I have a dual-processor hyperthreaded machine (so with four CPU
contexts), will a python program distribute threads over all four
logical processors?

I ask because I'm fairly sure that this *does* happen using the
threading extensions in MFC, and fairly sure that it *doesn't* when
using Java, so I don't see that the result is obvious for python.

Tom

[Duncan Booth]

Re: python threads on multi-CPU machines

Thomas Womack <twomack@chiark .greenend.org.u k> wrote in
news:vQn*aWb0p@ news.chiark.gre enend.org.uk:
[color=blue]
> If I have a dual-processor hyperthreaded machine (so with four CPU
> contexts), will a python program distribute threads over all four
> logical processors?
>
> I ask because I'm fairly sure that this *does* happen using the
> threading extensions in MFC, and fairly sure that it *doesn't* when
> using Java, so I don't see that the result is obvious for python.[/color]

The C implementation of Python uses a global interpreter lock that only
allows one thread to interpret bytecode at a time, so while the threads may
be distributed across multiple processors you will get little or no speedup
over a single processor. (If your threads spend most of their time in a
non-Python extension, they may be able to get some benefit from multiple
processors).

The only way to take advantage of multiple processors with Python is to run
at least one separate process for each processor. For example, I believe
Zope will take advantage of multiple processor systems if you run it in ZEO
client/server mode with several Zope client processes.

I believe that Jython simply does whatever the Java implementation does.

--
Duncan Booth duncan@rcp.co.u k
int month(char *p){return(1248 64/((p[0]+p[1]-p[2]&0x1f)+1)%12 )["\5\x8\3"
"\6\7\xb\1\x9\x a\2\0\4"];} // Who said my code was obscure?

[Andrew Dalke]

Re: python threads on multi-CPU machines

Thomas Womack:[color=blue]
> If I have a dual-processor hyperthreaded machine (so with four CPU
> contexts), will a python program distribute threads over all four
> logical processors?[/color]

Yes, unless you are CPU bound in the Python interpreter.
(CPU bound in an extension is fine, and I/O bound, as when
doing network is also fine.)

For a longer answer, see back threads concerning the global
interpreter lock and "free-threading".

Andrew
dalke@dalkescie ntific.com

[Kevin Cazabon]

Re: python threads on multi-CPU machines

> However, C extensions can release the[color=blue]
> GIL, and almost all I/O code in Python does that, so I/O-heavy programs
> will make good use of the SMP.[/color]

FYI, I'm also working on adding that functionality to the PIL library,
seeing as imaging operations can be fairly expensive too. It makes
these processes more "friendly" to other threads (and Tk) even on
single-CPU boxes, and allows taking advantage of multiple CPUs where
available.

I'll be posting links on the PIL mailing list when it's complete (just
needs exhaustive testing now... it'll be a while), and Fred has agreed
to merge it into the core distribution afterwards.

Kevin.

[Aahz]

Re: python threads on multi-CPU machines

In article <5a4226f0.03081 80945.34f96eca@ posting.google. com>,
Kevin Cazabon <kevin@cazabon. com> wrote:[color=blue]
>Aahz:[color=green]
>>
>> However, C extensions can release the GIL, and almost all I/O code
>> in Python does that, so I/O-heavy programs will make good use of the
>> SMP.[/color]
>
>FYI, I'm also working on adding that functionality to the PIL library,
>seeing as imaging operations can be fairly expensive too. It makes
>these processes more "friendly" to other threads (and Tk) even on
>single-CPU boxes, and allows taking advantage of multiple CPUs where
>available.[/color]

Cool! When you're done, I'd appreciate it if you could write up a short
summary of the problems you ran into and post it here. I'm interested
in pushing computational threading in Python, but I haven't written
enough C code to have any idea how it differs from other kinds of
threading issues.
--
Aahz (aahz@pythoncra ft.com) <*> http://www.pythoncraft.com/

This is Python. We don't care much about theory, except where it intersects
with useful practice. --Aahz

[Lothar Scholz]

Re: python threads on multi-CPU machines

Thomas Womack <twomack@chiark .greenend.org.u k> wrote in message news:<vQn*aWb0p @news.chiark.gr eenend.org.uk>. ..[color=blue]
> If I have a dual-processor hyperthreaded machine (so with four CPU
> contexts), will a python program distribute threads over all four
> logical processors?[/color]

Okay others told you already about the GIL.
[color=blue]
> I ask because I'm fairly sure that this *does* happen using the
> threading extensions in MFC, and fairly sure that it *doesn't* when
> using Java,[/color]

Of couse java *does* this. It's the number one reason for SUN to
develop and maintain Java: selling a reason to buy some of there
overpriced multiprocessor machines. Maybe you don't know anything
about Java.

[Paul McGuire]

Re: python threads on multi-CPU machines

Just one general caveat, based on some multi-thread code I worked on about a
year ago: if you want to support your users running your multithread code on
multi-CPU machines, you *must, must, must* stress test on multi-CPU
machines!

We had a very nasty intermittent bug (collision with threads in Tcl
interpreter) that I could *not* reproduce on my single-processor development
box, but which showed up in the field on the customer's quad-processor
machine.
Finally, after about a day and a half, I moved over to our dual processor
test machine, and the bug showed up after about 20 minutes of stress
testing. It turned out that the 3rd party thread library we were using was
built to be compatible with the Win98 (!) thread API - once I rebuilt to run
using the WinNT thread API, the window was closed, and the app ran great on
the multiCPU servers.

I'm sorry if this is off-topic, and I am fairly new in this newsgroup, but I
know we were very eager to push multiCPU machines on our customers, and we
did not realize what risk we were taking on.

Sincerely,
-- Paul McGuire
Austin, TX