On 25/10/2023 21:07, Andres Freund wrote:
It's not too awful to have it be in this order:
struct ResourceOwnerData {
ResourceOwner parent; /* 0 8 */
ResourceOwner firstchild; /* 8 8 */
ResourceOwner nextchild; /* 16 8 */
const char * name; /* 24 8 */
_Bool releasing; /* 32 1 */
_Bool sorted; /* 33 1 */
uint8 narr; /* 34 1 */
uint8 nlocks; /* 35 1 */
uint32 nhash; /* 36 4 */
uint32 capacity; /* 40 4 */
uint32 grow_at; /* 44 4 */
ResourceElem arr[32]; /* 48 512 */
/* --- cacheline 8 boundary (512 bytes) was 48 bytes ago --- */
ResourceElem * hash; /* 560 8 */
LOCALLOCK * locks[15]; /* 568 120 */
/* size: 688, cachelines: 11, members: 14 */
/* last cacheline: 48 bytes */
};
Requires rephrasing a few comments to document that the lenghts are separate
from the array / hashtable / locks, but otherwise...
Hmm, let's move 'capacity' and 'grow_at' after 'arr', too. They are only
needed together with 'hash'.
This reliably shows a decent speed improvement in my stress test [1], on the
order of 5%.
[1] pgbench running
DO $$ BEGIN FOR i IN 1..5000 LOOP PERFORM abalance FROM pgbench_accounts
WHERE aid = 17;END LOOP;END;$$;
I'm seeing similar results, although there's enough noise in the test
that I'm sure how real the would be across different tests.
At that point, the first array entry fits into the first cacheline. If we were
to move parent, firstchild, nextchild, name further down the struct, three
array entries would be on the first line. Just using the array presumably is a
very common case, so that might be worth it?
I got less clear performance results with this one, and it's also quite
possible it could hurt, if resowners aren't actually "used", just
released. Therefore it's probably not worth it for now.
You're assuming that the ResourceOwner struct begins at a cacheline
boundary. That's not usually true, we don't try to cacheline-align it.
So while it's helpful to avoid padding and to keep frequently-accessed
fields close to each other, there's no benefit in keeping them at the
beginning of the struct.
--
Heikki Linnakangas
Neon (https://neon.tech)
