On Thu, May 01, 2025 at 02:48:59PM -0400, Tom Lane wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Wed, Apr 30, 2025 at 4:53 AM Salvatore Dipietro >> <dipietro.salvat...@gmail.com> wrote: >>> we would like to propose the removal of the Instruction >>> Synchronization Barrier (isb) for aarch64 architectures. Based on our >>> testing on Graviton instances (m7g.16xlarge), we can see on average >>> over multiple iterations up to 12% better performance using PGBench >>> select-only and up to 9% with Sysbench oltp_read_only workloads. On >>> Graviton4 (m8g.24xlarge) results are up to 8% better using PGBench >>> select-only and up to 6% with Sysbench oltp_read_only workloads. >>> We have also tested it putting more pressure on the spin_delay >>> function, enabling pg_stat_statements.track_planning with PGBench >>> read-only [0] and, on average, the patch shows up to 27% better >>> performance on m6g.16xlarge and up to 37% on m7g.16xlarge. > >> I think you should make some kind of argument about why the previous >> conclusion was wrong, or why something's changed between then and now. > > TBH, those numbers are large enough that I flat out don't believe > them. As noted in the previous thread, we've managed to squeeze out > a lot of our dependencies on spinlock performance, via algorithmic > changes, migration to atomic ops, etc. So I think ten-percent-ish > improvement on a plain pgbench test just isn't very plausible. > We certainly didn't see that kind of effect when we were doing that > earlier round of testing --- we had to use a custom testing lashup > to get numbers that were outside the noise at all.
I'm not sure there's actually any hot spinlock involved in a regular select-only pgbench (unless perhaps pg_stat_statements was enabled or something). But I do know pg_stat_statements.track_planning stresses the spinlock code quite a bit, and commit 3d0b4b1 recently added a non-locking initial test in AArch64's TAS_SPIN, so I wonder if the ISB is still appropriate. It'd be interesting to see the performance difference of removing the ISB with and without commit 3d0b4b1 applied. -- nathan
