Re: slab allocator performance issues

Em seg., 19 de jul. de 2021 às 17:56, Andres Freund <andres(at)anarazel(dot)de>
escreveu:

> Hi,
>
> On 2021-07-18 19:23:31 +0200, Tomas Vondra wrote:
> > Sounds great! Thanks for investigating this and for the improvements.
> >
> > It might be good to do some experiments to see how the changes affect
> memory
> > consumption for practical workloads. I'm willing to spend soem time on
> that,
> > if needed.
>
> I've attached my changes. They're in a rough shape right now, but I
> think good enough for an initial look.
>
Hi Andres, I take a look.

Perhaps you would agree with me that in the most absolute of times, malloc
will not fail.
So it makes more sense to test:
if (ret != NULL)
than
if (ret == NULL)

What might help branch prediction.
With this change wins too, the possibility
to reduce the scope of some variable.

Example:
+static void * pg_noinline
+AllocSetAllocLarge(AllocSet set, Size size, int flags)
+{
+ AllocBlock block;
+ Size chunk_size;
+ Size blksize;
+
+ /* check size, only allocation path where the limits could be hit */
+ MemoryContextCheckSize(&set->header, size, flags);
+
+ AssertArg(AllocSetIsValid(set));
+
+ chunk_size = MAXALIGN(size);
+ blksize = chunk_size + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
+ block = (AllocBlock) malloc(blksize);
+ if (block != NULL)
+ {
+ AllocChunk chunk;
+
+ set->header.mem_allocated += blksize;
+
+ block->aset = set;
+ block->freeptr = block->endptr = ((char *) block) + blksize;
+
+ /*
+ * Stick the new block underneath the active allocation block, if
any,
+ * so that we don't lose the use of the space remaining therein.
+ */
+ if (set->blocks != NULL)
+ {
+ block->prev = set->blocks;
+ block->next = set->blocks->next;
+ if (block->next)
+ block->next->prev = block;
+ set->blocks->next = block;
+ }
+ else
+ {
+ block->prev = NULL;
+ block->next = NULL;
+ set->blocks = block;
+ }
+
+ chunk = (AllocChunk) (((char *) block) + ALLOC_BLOCKHDRSZ);
+ chunk->size = chunk_size;
+
+ return AllocSetAllocReturnChunk(set, size, chunk, chunk_size);
+ }
+
+ return NULL;
+}

regards,
Ranier Vilela