On Sat, Dec 6, 2025 at 3:04 PM Peter Geoghegan <pg@bowt.ie> wrote:
> My best guess is that the benefits I see come from eliminating a
> dependent load. Without the second patch applied, I see this
> disassembly for _bt_checkkeys:
>
> mov rax,QWORD PTR [rdi+0x38] ; Load scan->opaque
> mov r15d,DWORD PTR [rax+0x70] ; Load so->dir
>
> A version with the second patch applied still loads a pointer passed
> by the _bt_checkkeys caller (_bt_readpage), but doesn't have to chase
> another pointer to get to it. Maybe this significantly ameliorates
> execution port pressure in the cases where I see a speedup?
I found a way to further speed up the queries that the second patch
already helped with, following profiling with perf: if _bt_readpage
takes a local copy of scan->ignore_killed_tuples when first called,
and then uses that local copy within its per-tuple loop (instead of
using scan->ignore_killed_tuples directly), it gives me an additional
1% speedup over what I reported earlier today. In other words, the
range/BETWEEN pgbench variant I summarized earlier today goes from
being about 4.5% faster than master, to being about ~5.5% faster than
master. Testing has also shown that the ignore_killed_tuples
enhancement doesn't significantly change the picture with other types
of queries (such as the default pgbench SELECT).
In short, this ignore_killed_tuples change makes the second patch from
v1 more effective, seemingly by further ameliorating the same
bottleneck. Apparently accessing scan->ignore_killed_tuples created
another load-use hazard in the same tight inner loop (the per-tuple
_bt_readpage loop). Which matters with these queries, where we don't
need to do very much work per-tuple (_bt_readpage's pstate.startikey
optimization is as effective as possible here) and have quite a few
tuples (2,000 tuples) that need to be returned by each test query run.
Since this ignore_killed_tuples change is also very simple, and also
seems like an easy win, I think that it can be committed as part of
the second patch. Without it needing to wait for too much more
performance validation.
Attached are 2 text files showing pgbench output/summary info,
generated by my test script (both are from runs that took place within
the last 2 hours). One of these result sets just confirms what I
reported earlier on, with an unmodified v1 patchset. The other set of
results/file shows detailed results for the v1 patchset with the
ignore_killed_tuples change also applied, for the same pgbench
config/workload. This second file gives full details to back up my
"~5.5% faster than master" claim.
The pgbench script used for this is as follows:
\set aid random_exponential(1, 100000 * :scale, 3.0)
\set endrange :aid + 2000
SELECT abalance FROM pgbench_accounts WHERE aid between :aid AND :endrange;
I'm deliberately not attaching a new v2 for this ignore_killed_tuples
change right now. The first patch is a few hundred KBs, and I don't
want this email to get held up in moderation. Though I will attach the
ignore_killed_tuples change in its own patch, which I've also attached
(with a .txt extension, just to avoid confusing CFTester).
--
Peter Geoghegan
