Re: Make MemoryContextMemAllocated() more precise

On Mon, 2020-03-16 at 11:45 -0700, Jeff Davis wrote:
> AllocSet allocates memory for itself in blocks, which double in size
> up
> to maxBlockSize. So, the current block (the last one malloc'd) may
> represent half of the total memory allocated for the context itself.

Narrower approach that doesn't touch memory context internals:

If the blocks double up in size to maxBlockSize, why not just create
the memory context with a smaller maxBlockSize? I had originally
dismissed this as a hack that could slow down some workloads when
work_mem is large.

But we can simply make it proportional to work_mem, which makes a lot
of sense for an operator like HashAgg that controls its memory usage.
It can allocate in blocks large enough that we don't call malloc() too
often when work_mem is large; but small enough that we don't overrun
work_mem when work_mem is small.

I have attached a patch to do this only for HashAgg, using a new entry
point in execUtils.c called CreateWorkExprContext(). It sets
maxBlockSize to 1/16th of work_mem (rounded down to a power of two),
with a minimum of initBlockSize.

This could be a good general solution for other operators as well, but
that requires a bit more investigation, so I'll leave that for v14.

The attached patch is narrow and solves the problem for HashAgg nicely
without interfering with anything else, so I plan to commit it soon for
v13.

Regards,
    Jeff Davis