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
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