Carl J. Van Arsdall wrote: > Paul Rubin wrote: > >>"Carl J. Van Arsdall" <[EMAIL PROTECTED]> writes: >> >> >>>Processes seem fairly expensive from my research so far. Each fork >>>copies the entire contents of memory into the new process. >>> >> >>No, you get two processes whose address spaces get the data. It's >>done with the virtual memory hardware. The data isn't copied. The >>page tables of both processes are just set up to point to the same >>physical pages. Copying only happens if a process writes to one of >>the pages. The OS detects this using a hardware trap from the VM >>system. >> > > Ah, alright. So if that's the case, why would you use python threads > versus spawning processes? If they both point to the same address space > and python threads can't run concurrently due to the GIL what are they > good for? >
Well, of course they can interleave essentially independent computations, which is why threads (formerly "lightweight processes") were traditionally defined. Further, some thread-safe extension (compiled) libraries will release the GIL during their work, allowing other threads to execute simultaneously - and even in parallel on multi-processor hardware. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://holdenweb.blogspot.com Recent Ramblings http://del.icio.us/steve.holden -- http://mail.python.org/mailman/listinfo/python-list
