Обсуждение: Retrieve memory size allocated by libpq

Hello,

I would like to be able to retrieve the size of memory internally
allocated by libpq for a result. The reason is that we have a Ruby
wrapper that exposes libpq in Ruby. The problem is that Ruby's GC
doesn't know how much memory has been allocated by libpq, so no pressure
is applied to the GC when it should be. With this function we could
instruct the GC about the memory usage associated to each result object.

This issue has already been discussed in the following thread, with the
request to use custom malloc/realloc/free functions:

https://www.postgresql.org/message-id/flat/20170828172834.GA71455%40TC.local#20170828172834.GA71455@TC.local

Retrieving the allocated memory size is another approach to solve the
same base issue. However since the relation between memory consumption
and the particular result object is maintained, it can additionally be
used to provide diagnostic information to each object.

What do you think about adding such a function?

--
Kind Regards,
Lars

Attached is an updated patch of PQresultMemsize(). The implementation is
unchanged, but I added SGML documentation about this new function.

I'd be pleased about comments to adding this to libpq.

--
Kind Regards,
Lars

The following review has been posted through the commitfest application:
make installcheck-world:  tested, passed
Implements feature:       not tested
Spec compliant:           not tested
Documentation:            tested, passed

The implementation of PQresultMemsize is simple and correct. There are no warnings, and all tests passed.

I'll mark this patch as ready for committer

The new status of this patch is: Ready for Committer

Pavel Stehule <pavel.stehule@gmail.com> writes:
> The implementation of PQresultMemsize is simple and correct.

Hm, well, only for small values of "correct".  A quick look at PQclear
to see just what it clears shows that PGEvent objects associated with the
PGresult were overlooked.  While that's not hard to fix (and I did so),
there's a problem hiding behind that: we have no idea what sort of
"instance data" the external event procedures may have associated with
the PGresult.  In principle, any space occupied by such data needs to
be accounted for in this number.

This isn't an issue if the event procedure takes the lazy man's approach
of using PQresultAlloc to allocate such data.  That's recommendable anyway
since it saves having to write cleanup logic; and even if you don't do it
like that, the space involved may be negligible.  Still, I can foresee an
eventual need to let event procedures adjust the result-size number,
along the lines of PQincrementMemorySize(PGresult *res, size_t delta).
But that can probably wait till somebody actually requests it.

I didn't like the name "PQresultMemsize" much either, and changed it to
"PQresultMemorySize", which seems more in keeping with naming and
capitalization conventions elsewhere in libpq.

Pushed with those and some cosmetic changes.

            regards, tom lane