bug#68244: hash-table improvements

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

5 jan. 2024 kl. 16.41 skrev Dmitry Gutov <dmitry@gutov.dev>:

>> That's a good question and it all comes down to how we interpret `consing_until_gc`. Here we take the view that it should encompass all parts of an allocation and this seems to be consistent with existing code.
> 
> But the existing code used objects that would need to be collected by GC, right? And the new one, seemingly, does not.

But it does, similar to the same way that we deal with string data.

> So I don't quite see the advantage of increasing consing_until_gc then. It's like the difference between creating new strings and inserting strings into a buffer: new memory is used either way, but the latter doesn't increase consing.

Since we don't know exactly when objects die, we use object allocation as a proxy, assuming that on average A bytes die for every B bytes allocated and make an informed (and adjusted) guess as to what the A/B ratio might be. That is the basis for the GC clock.

Buffer memory is indeed treated differently and does not advance the GC clock as far as I can tell. Presumably the reasoning is that buffer size changes make a poor proxy for object deaths.

Of course we could reason that growing an existing hash table is also a bad proxy for object deaths, but the evidence for that is weak so I used the same metric as for other data structures just to be on the safe side.

This reminds me that the `gcstat` bookkeeping should probably include the hash-table ancillary arrays as well, since those counters are used to adjust the GC clock (see total_bytes_of_live_objects and consing_threshold). Will fix!

> It's great that the new hash tables are garbage-collected more easily and produce less garbage overall, but in a real program any GC cycle will have to traverse the other data structures anyway. So we might be leaving free performance gains on the table when we induce GC cycles while no managed allocations are done. I could be missing something, of course.

So could I, and please know that your questions are much appreciated. Are you satisfied by my replies above, or did I misunderstand your concerns?