On Wed, 20 Jan 2016 at 15:46 Victor Stinner <victor.stin...@gmail.com> wrote:
> Hi, > > 2016-01-20 22:18 GMT+01:00 Glenn Linderman <v+pyt...@g.nevcal.com>: > > On 1/20/2016 12:50 PM, Brett Cannon wrote: > >> > >> A global (shared between all dicts) unit64 ma_version is actually quite > >> reliable -- if a program does 1,000,000 dict modifications per second, > >> it would take it 600,000 years till wrap-around. > > I think that Yury found a bug in FAT Python. I didn't test the case > when the builtins dictionary is replaced after the definition of the > function. To be more concrete: when a function is executed in a > different namespace using exec(code, namespace). That's why I like the > PEP process, it helps to find all issues before going too far :-) > > I like the idea of global counter for dictionary versions. It means > that the dictionary constructor increases this counter instead of > always starting to 0. > > FYI a fat.GuardDict keeps a strong reference to the dictionary. For > some guards, I hesitated to store the object identifier and/or using a > weak reference. An object identifier is not reliable because the > object can be destroyed and a new object, completly different, or of > the same type, can get the same identifier. > > > But would invalidate everything, instead of just a fraction of things, on > > every update to anything that is monitored... > > I don't understand this point. > I think Glenn was assuming we had a single, global version # that all dicts shared *without* having a per-dict version ID. The key thing here is that we have a global counter that tracks the number of mutations for *all* dictionaries but whose value we store as a *per-dictionary* value. That ends up making the version ID inherently both a token representing the state of any dict but also the uniqueness of the dict since no two dictionaries will ever have the same version ID. > > In short, the guard only has to compare two 64 bit integers in the > fast-path, when nothing changed. For a namespace, it means that no > value was replaced in this namespace. > > If a different namespace is modified, the version of the watched > namespace does not change, so we are still in the fast-path. > > If a value is replaced in the watched namespace, but not the watched > variable, we have to take a slow-path, hopefully only once. > > The worst case is when a value different than the watched value is > modified between each guard check. In this case, we always need a dict > lookup. An heuristic can be chosen to decide to give up after N tries. > Currently, fat.GuardDict always retries. > Does "retries" mean "check if the value really changed, and if it hasn't then just update the version ID the guard checks"?
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