Re: CSN snapshots in hot standby

On Tue, Dec 3, 2024 at 9:25 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>
> On 20/11/2024 15:33, John Naylor wrote:
> I did find one weird thing that makes a big difference: I originally
> used AllocSetContextCreate(..., ALLOCSET_DEFAULT_SIZES) for the radix
> tree's memory context. With that, XidInMVCCSnapshot() takes about 19% of
> the CPU time in that test. When I changed that to ALLOCSET_SMALL_SIZES,
> it falls down to the 4% figure. And weird enough, in both cases the time
> seems to be spent in the malloc() call from SlabContextCreate(), not
> AllocSetContextCreate(). I think doing this particular mix of large and
> small allocations with malloc() somehow poisons its free list or
> something. So this is probably heavily dependent on the malloc()
> implementation. In any case, ALLOCSET_SMALL_SIZES is clearly a better
> choice here, even without that effect.

Hmm, interesting. That passed context is needed for 4 things:
1. allocated values (not used here for 64-bit, and 32-bit could be
made to work the same way)
2. iteration state (not used here)
3. a convenient place to put slab child contexts so we can free them easily
4. a place to put the "control object" -- this is really only needed
for shared memory and I have a personal todo to  embed it rather than
allocate it for the local memory case.

Removing the need for a passed context for callers that don't need it
is additional possible future work.

Anyway, 0005 looks good to me.

--
John Naylor
Amazon Web Services