On 2025-Jan-31, Dmitry Koterov wrote: > PG "startup recovering" eats up a lot of CPU (like 65 %user and 30 %sys), > which is a little surprising (what is it doing with all those CPU cycles? > it looked like WAL replay should be more IO bound than CPU bound?). > > Running "perf top -p <pid>", it shows this: > > Samples: 1M of event 'cycles:P', 4000 Hz, Event count (approx.): > 18178814660 lost: 0/0 drop: 0/0 > Overhead Shared Object Symbol > 16.63% postgres [.] hash_search_with_hash_value
Yeah, I noticed that this function was showing high in some profiles a couple of days ago as well. Looking now at hashfn.c (hash_bytes_uint32 there is the function involved in the buffer mapping hash table), the comments state that we're updated to Bob Jenkins code from 2006, but there's a version in his website that (if I read correctly) is twice as fast as what we're using now: http://burtleburtle.net/bob/hash/spooky.html Apparently this code in our repo is mostly unchanged since commit 1f559b7d3aa4, in 2007. He mentions that on Intel chips, Google's CityHash is faster; but they in turn claim that the difference is small on Intel chips and that Jenkins' hash is better on AMD chips. https://github.com/google/cityhash Anyway if you wanted to try your luck at improving things, here's your chance. -- Álvaro Herrera 48°01'N 7°57'E — https://www.EnterpriseDB.com/
