Re: Change GUC hashtable to use simplehash?

On Mon, Nov 20, 2023 at 5:54 AM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
>
> Attached are a bunch of tiny patches and some perf numbers based on
> simple test described here:
>
> https://www.postgresql.org/message-id/04c8592dbd694e4114a3ed87139a7a04e4363030.camel%40j-davis.com

I tried taking I/O out, like this, thinking the times would be less variable:

cat bench.sql
select 1 from generate_series(1,500000) x(x), lateral (SELECT
inc_ab(x)) a offset 10000000;

(with turbo off)
pgbench -n -T 30 -f bench.sql -M prepared

master:
latency average = 643.625 ms
0001-0005:
latency average = 607.354 ms

...about 5.5% less time, similar to what Jeff found.

I get a noticeable regression in 0002, though, and I think I see why:

guc_name_hash(const char *name)
{
- uint32 result = 0;
+ const unsigned char *bytes = (const unsigned char *)name;
+ int blen = strlen(name);

The strlen call required for hashbytes() is not free. The lack of
mixing in the (probably inlined after 0001) previous hash function can
remedied directly, as in the attached:

0001-0002 only:
latency average = 670.059 ms

0001-0002, plus revert hashbytes, add finalizer:
latency average = 656.810 ms

-#define SH_EQUAL(tb, a, b) (guc_name_compare(a, b) == 0)
+#define SH_EQUAL(tb, a, b) (strcmp(a, b) == 0)

Likewise, I suspect calling out to the C library is going to throw
away some of the gains that were won by not needing to downcase all
the time, but I haven't dug deeper.