Re: Reducing the chunk header sizes on all memory context types

On 8/30/22 04:31, David Rowley wrote:
> On Tue, 30 Aug 2022 at 13:55, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I wonder if slab ought to artificially bump up such requests when
>> MEMORY_CONTEXT_CHECKING is enabled, so there's room for a sentinel.
>> I think it's okay for aset.c to not do that, because its power-of-2
>> behavior means there usually is room for a sentinel; but slab's
>> policy makes it much more likely that there won't be.
> 
> I think it's fairly likely that small allocations are a power of 2,
> and I think most of our allocates are small, so I imagine that if we
> didn't do that for aset.c, we'd miss out on most of the benefits.
> 

Yeah. I think we have a fair number of "larger" allocations (once you
get to ~100B it probably won't be a 2^N), but we may easily miss whole
sections of allocations.

I guess the idea was to add a sentinel only when there already is space
for it, but perhaps that's a bad tradeoff limiting the benefits. Either
we add the sentinel fairly often (and then why not just add it all the
time - it'll need a bit more space), or we do it only very rarely (and
then it's a matter of luck if it catches an issue). Considering we only
do this with asserts, I doubt the extra bytes / CPU is a major issue,
and a (more) reliable detection of issues seems worth it. But maybe I
underestimate the costs. The only alternative seems to be valgrind, and
that's way costlier, though.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company