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

On Tue, Aug 9, 2022 at 8:53 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> I think the patch is now starting to take shape. I've added it to the
> September commitfest [1].

This is extremely cool. The memory savings are really nice. And I also
like this:

# Additionally, this commit completely changes the rule that pointers must
# be directly prefixed by the owning memory context and instead, we now
# insist that they're directly prefixed by an 8-byte value where the least
# significant 3-bits are set to a value to indicate which type of memory
# context the pointer belongs to. Using those 3 bits as an index to a new
# array which stores the methods for each memory context type, we're now
# able to pass the pointer given to functions such as pfree and repalloc to
# the function specific to that context implementation to allow them to
# devise their own methods of finding the memory context which owns the
# given allocated chunk of memory.

That seems like a good system.

This part of the commit message might need to be clarified:

# We also add a restriction that block sizes for all 3 of the memory
# allocators cannot be 1GB or larger. We would be unable to store the
# number of bytes that the block is offset from the chunk stored beyond this
#1GB boundary on any block that was larger than 1GB.

Earlier in the commit message, you say that allocations of 1GB or more
are stored in dedicated blocks. But here you say that blocks can't be
more than 1GB. Those statements seem to contradict each other. I guess
you mean block sizes for blocks that contain chunks, or something like
that?

--
Robert Haas
EDB: http://www.enterprisedb.com