On Sat, 6 Apr 2019 at 16:03, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I'd also point out that this is hardly the only place where we've
> seen hash_seq_search on nearly-empty hash tables become a bottleneck.
> So I'm not thrilled about attacking that with one-table-at-time patches.
> I'd rather see us do something to let hash_seq_search win across
> the board.
Rewinding back to mid-Feb:
You wrote:
> My own thought about how to improve this situation was just to destroy
> and recreate LockMethodLocalHash at transaction end (or start)
> if its size exceeded $some-value. Leaving it permanently bloated seems
> like possibly a bad idea, even if we get rid of all the hash_seq_searches
> on it.
Which I thought was an okay idea. I think the one advantage that
would have over making hash_seq_search() faster for large and mostly
empty tables is that over-sized hash tables are just not very cache
efficient, and if we don't need it to be that large then we should
probably consider making it smaller again.
I've had a go at implementing this and using Amit's benchmark the
performance looks pretty good. I can't detect any slowdown for the
general case.
master:
plan_cache_mode = auto:
$ pgbench -n -M prepared -T 60 -f select.sql postgres
tps = 9373.698212 (excluding connections establishing)
tps = 9356.993148 (excluding connections establishing)
tps = 9367.579806 (excluding connections establishing)
plan_cache_mode = force_custom_plan:
$ pgbench -n -M prepared -T 60 -f select.sql postgres
tps = 12863.758185 (excluding connections establishing)
tps = 12787.766054 (excluding connections establishing)
tps = 12817.878940 (excluding connections establishing)
shrink_bloated_locallocktable.patch:
plan_cache_mode = auto:
$ pgbench -n -M prepared -T 60 -f select.sql postgres
tps = 12756.021211 (excluding connections establishing)
tps = 12800.939518 (excluding connections establishing)
tps = 12804.501977 (excluding connections establishing)
plan_cache_mode = force_custom_plan:
$ pgbench -n -M prepared -T 60 -f select.sql postgres
tps = 12763.448836 (excluding connections establishing)
tps = 12901.673271 (excluding connections establishing)
tps = 12856.512745 (excluding connections establishing)
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services