On 29 July 2010 18:02, Maciej Fijalkowski <fijall@gmail.com> wrote:
On Thu, Jul 29, 2010 at 9:57 AM, William Leslie <william.leslie.ttg@gmail.com> wrote:
I claim that there are two alternatives in the face of one thread mutating an object and the other observing:
0. You can give up consistency and do fine-grained locking, which is reasonably fast but error prone, or 1. Expect python to handle all of this for you, effectively not making a change to the memory model. You could do this with implicit per-object locks which might be reasonably fast in the absence of contention, but not when several threads are trying to use the object.
Queues already are in a sense your per-object-lock, one-thread-mutating, but usually one thread has acquire semantics and one has release semantics, and that combination actually works. It's when you expect to have a full memory barrier that is the problem.
Come to think of it, you might be right Kevin: as long as only one thread mutates the object, the mutating thread never /needs/ to acquire, as it knows that it has the latest revision.
Have I missed something?
-- William Leslie
So my question is why you think 1. is really expensive (can you find evidence). I don't see what is has to do with cache misses. Besides, in python you cannot guarantee much about mutability of objects. So you don't know if object passed in a queue is mutable or not, unless you restrict yourself to some very simlpe types (in which case there is no shared memory, since you only pass immutable objects).
If task X expects that task Y will mutate some object it has, it needs to go back to the source for every read. This means that if you do use mutation of some shared object for communication, it needs to be synchronised before every access. What this means for us is that every read from a possibly mutable object requires an acquire, and every write requires a release. It's as if every reference in the program is implemented with a volatile pointer. Even if the object is never mutated, there can be a lot of unnecessary bus chatter waiting for MESI to tell us so. -- William Leslie