Re: PATCH: two slab-like memory allocators

Hi,

On 2017-02-27 15:11:40 +0100, Tomas Vondra wrote:
> > I've not yet reviewed the generational allocator yet, but during these
> > measurements I get:
> > postgres[3970][1]=# select count(*) FROM pg_logical_slot_get_changes('ttt', NULL, NULL);
> > WARNING: 01000: problem in Generation Tuples: number of free chunks 0 in block 0x55d011ef10f0 exceeds 7234 allocated
> > LOCATION: GenerationCheck, generation.c:693
> > WARNING: 01000: problem in Generation Tuples: number of free chunks 0 in block 0x55d01023eba0 exceeds 65532 allocated
> > LOCATION: GenerationCheck, generation.c:693
> > WARNING: 01000: problem in Generation Tuples: number of free chunks 0 in block 0x55d00d7fb870 exceeds 65532 allocated
> > LOCATION: GenerationCheck, generation.c:693
> > WARNING: 01000: problem in Generation Tuples: number of free chunks 0 in block 0x55d00cde17b0 exceeds 65531 allocated
> > LOCATION: GenerationCheck, generation.c:693
> >
> > that seems to occur when there's currently in-progress transactions when
> > finishing decoding:
> >
> ...
> >
> > could it be that the test's condition is inverted?
> >
>
> Yeah, that seems like the culprit - the condition seems wrong. I wonder why
> I haven't seen it during my tests, though ...

I suspect it's because your tests only triggered a memory context reset
when it was empty... But I ran decoding while a concurrent write
transaction was ongoing...

> > I'll work on getting slab committed first, and then review / edit /
> > commit generation.c later. One first note there is that I'm wondering
> > if generation.c is a too generic filename.

> Naming things is hard.

Indeed. I was thinking of genalloc, but that might be understood as
general, rather generational...

Greetings,

Andres Freund