Re: Using per-transaction memory contexts for storing decoded tuples

On Fri, 20 Sept 2024 at 17:46, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Sep 20, 2024 at 5:13 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> > In general, it's a bit annoying to have to code around this
> > GenerationContext fragmentation issue.
>
> Right, and I am also slightly afraid that this may not cause some
> regression in other cases where defrag wouldn't help.

Yeah, that's certainly a possibility. I was hoping that
MemoryContextMemAllocated() being much larger than logical_work_mem
could only happen when there is fragmentation, but certainly, you
could be wasting effort trying to defrag transactions where the
changes all arrive in WAL consecutively and there is no
defragmentation. It might be some other large transaction that's
causing the context's allocations to be fragmented. I don't have any
good ideas on how to avoid wasting effort on non-problematic
transactions. Maybe there's something that could be done if we knew
the LSN of the first and last change and the gap between the LSNs was
much larger than the WAL space used for this transaction. That would
likely require tracking way more stuff than we do now, however.

With the smaller blocks idea, I'm a bit concerned that using smaller
blocks could cause regressions on systems that are better at releasing
memory back to the OS after free() as no doubt malloc() would often be
slower on those systems. There have been some complaints recently
about glibc being a bit too happy to keep hold of memory after free()
and I wondered if that was the reason why the small block test does
not cause much of a performance regression. I wonder how the small
block test would look on Mac, FreeBSD or Windows. I think it would be
risky to assume that all is well with reducing the block size after
testing on a single platform.

David