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Re: Improving the memory allocator

From: Andres Freund <andres(at)anarazel(dot)de>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Improving the memory allocator
Date: 2011-04-26 00:24:00
Message-ID: 201104260224.00647.andres@anarazel.de (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tuesday, April 26, 2011 01:24:20 AM Robert Haas wrote:
> On Mon, Apr 25, 2011 at 5:45 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > [ lots of awesome test results ]
> 
> Very interesting work.  I think it's interesting that there is so much
> allocation happening inside MessageContext; I barely knew that
> existed, let alone that it was a hotspot for memory allocation.
Yes, I was surprised as well. I haven't checked yet what the callsites for 
that are.
As I just wrote to TOm the hotspots are wildly different if you use slightly 
more complex statements. Which doesn't mean its not worthy of improvement ;)

> I
> think it would be interesting to break out the calls to new_list() by
> caller.  It's been vaguely bothering me for a long time that we store
> so many things in using List structures.  That is not a particularly
> efficient representation, because a List of N nodes requires 2N+1
> memory allocations - N nodes, N ListCell objects, and the List object
> itself.  It might be worth experimenting with some kind of array/list
> hybridy thing, like this:
> ...
While I aggree that that might be beneficial I think in this case nearly all 
of the lists are of 1 element length because its callers seem to look mostly 
like:
{{{
List *
lappend(List *list, void *datum)
{
	Assert(IsPointerList(list));

	if (list == NIL)
		list = new_list(T_List);
	else
		new_tail_cell(list);

	lfirst(list->tail) = datum;
	check_list_invariants(list);
	return list;
}
}}}

Hm. Seems worthy of some further investigation...

Thanks,

Andres

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