Re: Bad COMPACT_ALLOC_CHUNK size in tsearch/spell.c?

On Tue, Apr 26, 2011 at 1:48 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I wrote:
>> A different angle of attack on the issue is that aset.c's use of
>> exact-power-of-2 sizes for both malloc requests and the available space
>> in chunks is inefficient when maxBlockSize is constrained to be not much
>> larger than common chunk request sizes.  Maybe we should try to fix that
>> instead of asking other code to choose request sizes to dodge the
>> inefficiency.
>
> After chewing on that thought for a bit, it seems like an easy fix is to
> modify AllocSetContextCreate (around line 390 in HEAD's aset.c) so that
> allocChunkLimit is not just constrained to be less than maxBlockSize,
> but significantly less than maxBlockSize --- say an eighth or so.  When
> using ALLOCSET_SMALL_MAXSIZE this would result in requests of 1K or more
> being treated as separate malloc blocks, in turn guaranteeing that not
> more than 1K of each 8K request block could go to waste in the face of a
> stream of allocChunkLimit-sized requests.
>
> Of course, this might just result in the fragmentation problem getting
> tossed over the fence into malloc's playground ... but offhand it seems
> that it would remove the issue that spell.c is concerned about.

well, +1 on any solution that doesn't push having to make assumptions
about the allocator from the outside. your fix seems to nail it
without having to tinker around with the api which is nice. (plus you
could just remove the comment).

Some perfunctory probing didn't turn up any other cases like this.

merlin