[Python-Dev] Reworking the GIL (original) (raw)

Jeffrey Yasskin jyasskin at gmail.com
Mon Nov 2 18:56:11 CET 2009

On Mon, Nov 2, 2009 at 4:15 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:

Martin v. Löwis <martin v.loewis.de> writes:

[gildroprequest] Even if it is read from memory, I still wonder what might happen on systems that require explicit memory barriers to synchronize across CPUs. What if CPU 1 keeps reading a 0 value out of its cache, even though CPU 1 has written an 1 value a long time ago?

IIUC, any (most?) pthread calls would cause synchronization in that case, which is why applications that also use locks for reading: http://www.opengroup.org/onlinepubs/009695399/basedefs/xbdchap04.html#tag0410 Of course, on x86, you won't see any issues, because it's cache-coherent anyway. I think there are two things here: - all machines Python runs on should AFAIK be cache-coherent: CPUs synchronize their views of memory in a rather timely fashion. - memory ordering: writes made by a CPU can be seen in a different order by another CPU (i.e. CPU 1 writes A before B, but CPU 2 sees B written before A). I don't see how this can apply to gildroprequest, since it's a single variable (and, moreover, only a single bit of it is significant). (there's an explanation of memory ordering issues here: http://www.linuxjournal.com/article/8211) As a side note, I remember Jeffrey Yasskin trying to specify an ordering model for Python code (see http://code.google.com/p/unladen-swallow/wiki/MemoryModel).

Note that that memory model was only for Python code; the C code implementing it is subject to the C memory model, which is weaker (and not even fully defined until the next C standard comes out).

To be really safe, we ought to have a couple primitives implementing "atomic" rather than just "volatile" instructions, but until then a signal that's just saying "do something" rather than "here's some data you should look at" should be ok as a volatile int.

I'd like to look at the patch in detail, but I can't guarantee that I'll get to it in a timely manner. I'd say check it in and let more threading experts look at it in the tree. We've got some time before a release for people to fix problems and make further improvements. +1 to Martin's request for detailed documentation though. :)

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