Re: memory management suggestion

Where are we on this?

>
> I start detail study of PG's memory management (because, I want remove
> prepared query-cache to shmem (more is in my old discussion with Jan)).
>
> I see current code in the aset.c and I found small non-effective memory
> usage.
>
>
> Description:
>
> The postgresql use blocks for allocation. These blocks are inward
> organized/split via chunks.
>
> If a palloc() wants memory:
> 1) try use some chunk in a freelist of chunks
> 2) try use free space in an actual block
> 3) if wanted space is large - allocate specific one-block-for-one-chunk
> 4) if previous options are not possible allocate new block
>
> A problem:
>
> - if use option 4) and already exist (old) block (but space in this block
> is less than wanted space) current algorithm _skip_ and not _use_ this small
> space in old block. For a detail see the 'else' on line 327 in aset.c.
>
> I test it and average is 8-10b per a block (but max is 1000b) - large is
> this space if a palloc() wants bigger space.
>
> A solution:
>
> Create from this non-used residual space chunk and remove it into free
> chunk list.
>
>
> Comments?
>
> Karel
>
>
> -----> A tested patch (hmm, we are freeze, possible for 7.0.?):
>
> *** aset.orig.c Thu Apr 13 18:33:45 2000
> --- aset.c Thu Apr 20 18:45:50 2000
> ***************
> *** 323,326 ****
> --- 323,346 ----
> else
> {
> + int oldfree = set->blocks->endptr - set->blocks->freeptr;
> +
> + /*
> + * Try create from residual space in block free chunk
> + */
> + if (oldfree > MAXALIGN(1) + ALLOC_CHUNKHDRSZ) {
> +
> + int x_fidx = AllocSetFreeIndex(oldfree - ALLOC_CHUNKHDRSZ );
> +
> + chunk = (AllocChunk) (set->blocks->freeptr);
> + chunk->size = oldfree - ALLOC_CHUNKHDRSZ;
> +
> + /* put chunk into freelist */
> + chunk->aset = (void *) set->freelist[x_fidx];
> + set->freelist[x_fidx] = chunk;
> +
> + /* unset free space in block */
> + set->blocks->freeptr = set->blocks->endptr;
> + }
> +
> /* Get size of prior block */
> blksize = set->blocks->endptr - ((char *) set->blocks);
>
>
>
>

--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026