(I just realized that we started discussing details of immortal objects in the wrong thread -- this is Eric's overview thread, there's a separate thread on immortal objects. But alla, I'll respond here below.)
On Wed, Dec 15, 2021 at 5:05 PM Neil Schemenauer <n...@python.ca> wrote: > On 2021-12-15 2:57 p.m., Guido van Rossum wrote: > > But as long as the imbalance is less than 0x_2000_0000, the refcount will > remain in the inclusive range [ 0x_4000_0000 , 0x_7FFF_FFFF ] and we can > test for immortality by testing a single bit: > > if (o->ob_refcnt & 0x_4000_0000) > > Could we have a full GC pass reset those counts to make it even more > unlikely to get out of bounds? > Maybe, but so far these are all immutable singletons that aren't linked into the GC at all. Of course we could just add extra code to the GC code that just resets all these refcounts, but since there are ~260 small integers that might slow things down more than we'd like. More testing is required. Maybe we can get away with doing nothing on 64-bit machines but we'll have to slow down a tad for 32-bit -- that would be acceptable (since the future is clearly 64-bit). > Allocating immortal objects from a specific memory region seems like > another idea worth pursuing. It seems mimalloc has the ability to allocate > pools aligned to certain large boundaries. That takes some platform > specific magic. If we can do that, the test for immortality is pretty > cheap. However, if you can't allocate them at a fixed region determined at > compile time, I don't think you can match the performance of the code > above. Maybe it helps that you could determine immortality by looking at > the PyObject pointer and without loading the ob_refcnt value from memory? > You would do something like: > > if (((uintptr_t)o) & _Py_immortal_mask) > > The _Py_immortal_mask value would not be known at compile time but would > be a global constant. So, it would be cached by the CPU. > Very clever. -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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