Re: Re: [PATCH] unified frontend support for pg_malloc et al and palloc/pfree mulation (was xlogreader-v4)

I wrote:
> I then applied the palloc.h and mcxt.c hunks of your patch and rebuilt.
> Now I get an average runtime of 16666 ms, a full 2% faster, which is a
> bit astonishing, particularly because the oprofile results haven't moved
> much:

I studied the assembly code being generated for palloc(), and I believe
I see the reason why it's a bit faster: when there's only a single local
variable that has to survive over the elog call, gcc generates a shorter
function entry/exit sequence.  I had thought of proposing that we code
palloc() like this:

void *
palloc(Size size)
{   MemoryContext context = CurrentMemoryContext;
   AssertArg(MemoryContextIsValid(context));
   if (!AllocSizeIsValid(size))       elog(ERROR, "invalid memory alloc request size %lu",            (unsigned long)
size);
   context->isReset = false;
   return (*context->methods->alloc) (context, size);
}

but at least on this specific hardware and compiler that would evidently
be a net loss compared to direct use of CurrentMemoryContext.  I would
not put a lot of faith in that result holding up on other machines
though.

In any case this doesn't explain the whole 2% speedup, but it probably
accounts for palloc() showing as slightly cheaper than
MemoryContextAlloc had been in the oprofile listing.
        regards, tom lane