From past few weeks, we were facing some performance degradation in the read-only performance bench marks in high-end machines. My colleague Mithun, has tried by reverting commit ac1d794 which seems to degrade the performance in HEAD on high-end m/c's as reported previously[1], but still we were getting degradation, then we have done some profiling to see what has caused it and we found that it's mainly caused by spin lock when called via pin/unpin buffer and then we tried by reverting commit 6150a1b0 which has recently changed the structures in that area and it turns out that reverting that patch, we don't see any degradation in performance. The important point to note is that the performance degradation doesn't occur every time, but if the tests are repeated twice or thrice, it is easily visible.
m/c details
IBM POWER-8
24 cores,192 hardware threads
RAM - 492GB
Non-default postgresql.conf settings-
shared_buffers=16GB
max_connections=200
min_wal_size=15GB
max_wal_size=20GB
checkpoint_timeout=900
maintenance_work_mem=1GB
checkpoint_completion_target=0.9
scale_factor - 300
Performance at commit 43cd468cf01007f39312af05c4c92ceb6de8afd8 is 469002 at 64-client count and then at 6150a1b08a9fe7ead2b25240be46dddeae9d98e1, it went down to 200807. This performance numbers are median of 3 15-min pgbench read-only tests. The similar data is seen even when we revert the patch on latest commit. We have yet to perform detail analysis as to why the commit 6150a1b08a9fe7ead2b25240be46dddeae9d98e1 lead to degradation, but any ideas are welcome.