Scaling PostgreSQL at multicore Power8

Hello hackers

Recently, we were given access to the test server is IBM, 9119-MHE with 8 CPUs
* 8 cores * 8 threads. We decided to take advantage of this and to find
bottlenecks for read scalability (pgbench -S).

All detail you can read here:
http://www.postgrespro.ru/blog/pgsql/2015/08/30/p8scaling

Performance 9.4 stopped growing after 100 clients, and 9.5 / 9.6 stopped after
150 (at 4 NUMA nodes). After research using pref we saw that inhibits
ProcArrayLock in GetSnaphotData. But inserting the stub instead of
GetSnapshotData not significantly increased scalability. Trying to find the
bottleneck with gdb, we found another place. We have noticed s_lock in
PinBuffer and UnpinBuffer. For the test we rewrited PinBuffer and UnpinBuffer
by atomic operations and we liked the result. Degradation of performance
almost completely disappeared, and went scaling up to 400 clients (4 NUMA
nodes with 256 "CPUs").

To scale Postgres for large NUMA machine must be ported to the atomic
operations bufmgr. During our tests, we no found errors in our patch, but most
likely it is not true and this patch only for test.

Who has any thoughts?

--
YUriy Zhuravlev
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company