Fix comment for max_cached_tuplebufs definition

On Fri, Feb 7, 2020 at 02:49:14PM +0530, Kuntal Ghosh wrote:

Hello Hackers,

While working on some issue in logical decoding, I found some
inconsistencies in the comment for defining max_cached_tuplebufs in
reorderbuffer.c. It only exists till PG10 because after that the
definition got removed by the generational memory allocator patch. The
variable is defined as follows in reorderbuffer.c:
static const Size max_cached_tuplebufs = 4096 * 2; /* ~8MB */

And it gets compared with rb->nr_cached_tuplebufs in
ReorderBufferReturnTupleBuf as follows:
if (tuple->alloc_tuple_size == MaxHeapTupleSize &&
rb->nr_cached_tuplebufs < max_cached_tuplebufs)

{
rb->nr_cached_tuplebufs++;
}

So, what this variable actually tracks is 4096 * 2 times
MaxHeapTupleSize amount of memory which is approximately 64MB. I've
attached a patch to modify the comment.

But, I'm not sure whether the intention was to keep 8MB cache only. In
that case, I can come up with another patch.

Yes, I see you are correct, since each tuplebuf is MaxHeapTupleSize.
Patch applied from PG 9.5 to PG 10. Thanks.

--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EnterpriseDB https://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +