[pypy-dev] Thinking about the GIL

[Ben.Young at sungard.com]

> -----Original Message-----
> From: Leonardo Santagada [mailto:santagada at gmail.com]
> Sent: 15 March 2011 17:25
> To: Young, Ben
> Cc: benjamin at python.org; pypy-dev at codespeak.net
> Subject: Re: [pypy-dev] Thinking about the GIL
> 
> On Tue, Mar 15, 2011 at 11:36 AM,  <Ben.Young at sungard.com> wrote:
> >
> >
> >> -----Original Message-----
> >> From: pypy-dev-bounces at codespeak.net [mailto:pypy-dev-
> >> bounces at codespeak.net] On Behalf Of Benjamin Peterson
> >> Sent: 15 March 2011 14:18
> >> To: Young, Ben
> >> Cc: pypy-dev at codespeak.net
> >> Subject: Re: [pypy-dev] Thinking about the GIL
> >>
> >> 2011/3/15  <Ben.Young at sungard.com>:
> >> >
> >> >
> >> >> -----Original Message-----
> >> >> From: pypy-dev-bounces at codespeak.net [mailto:pypy-dev-
> >> >> bounces at codespeak.net] On Behalf Of Benjamin Peterson
> >> >> Sent: 14 March 2011 19:29
> >> >> To: Timothy Baldridge
> >> >> Cc: PyPy Dev
> >> >> Subject: Re: [pypy-dev] Thinking about the GIL
> >> >>
> >> >> 2011/3/14 Timothy Baldridge <tbaldridge at gmail.com>:
> >> >> > They may not be thread-safe, but as far as a program goes, do
> we
> >> >> > really care? If I have two threads adding items to the same
> list,
> >> >> > should I really be expecting the interpreter to keep things
> >> > straight?
> >> >> > What's wrong with forcing the user to lock structures before
> >> editing
> >> >> > them? This is something that Java, and C#, and C++ all require.
> >> >>
> >> >> Yes, the Python interpreter should never crash because of user
> >> >> mistakes.
> >> >>
> >> >
> >> > C# structures aren't thread-safe, but you can't crash the CLR by
> >> using
> >> > them in a multithreaded manner
> >>
> >> The primitive data structures must be thread-safe, though.
> >>
> >
> > No, just cleverly designed with the memory model in mind I believe
> (at
> > least there's no locking or even atomic synchronisation), and you can
> > get errors, it just wont bring down the VM
> 
> That is what I think he meant. The only way that I know for a vm to be
> thread-safe is its internal structures to be thread-safe. The exposed
> C# structures might not be, but the VM ones have to.
> 

Yes, I guess so. I just meant that you won't run into scaling issues in C# because the core VM is locking all over the place or anything like that. In Python (and PyPy) the problem is worse because the VM is implemented using many of the same structures as are exposed at app level, whereas the CLR is much lower level and so is probably easier to write in a thread safe manner. 

Having said that, if the PyPy GCs were thread-safe, how many other structures would need to be changed to allow a free threaded python that's "use at own risk" e.g. one that's allowed to crash if you do silly things as I guess most people would be happy with that. The import machinery would need to be thread safe but how much more?

Cheers,
Ben

> --
> Leonardo Santagada