Performance hickups after Postgres 9.4 upgrade

After upgrading our database from 9.3.5 to 9.4.1 last night, the server suffers from high CPU spikes. The upgrade was done with pg_dump. So the database was converted to SQL and then imported into 9.4.

During the CPU spikes, there are a lot of these messages in the logs:

process X still waiting for ExclusiveLock on extension of relation Y of database Z 
after 1036.234 ms

And:

process X acquired ExclusiveLock on extension of relation Y of database Z
after 2788.050 ms

What looks suspicious is that there are sometimes several "acquired" messages for the exact same relation number in the exact same millisecond.

Why would Postgres grow a table twice in the same millisecond? Could it be an index with a high fill factor?

Any suggestions on how to approach this issue are welcome.

P.S. I've also asked this question on the Postgres mailing list, if that's not okay let me know.

The problem had to do with a kernel feature called Transparent Huge Pages (THP.) You can diagnose this with perf top:

59.73%       postmaster  [kernel.kallsyms]      [k] compaction_alloc
  1.31%       postmaster  [kernel.kallsyms]      [k] _spin_lock
  0.94%       postmaster  [kernel.kallsyms]      [k] __reset_isolation_suitable
  0.78%       postmaster  [kernel.kallsyms]      [k] compact_zone
  0.67%       postmaster  [kernel.kallsyms]      [k] get_pageblock_flags_group
  0.64%       postmaster  [kernel.kallsyms]      [k] copy_page_c
  0.48%           :13410  [kernel.kallsyms]      [k] compaction_alloc
  0.45%           :13465  [kernel.kallsyms]      [k] compaction_alloc
  0.45%       postmaster  [kernel.kallsyms]      [k] clear_page_c
  0.44%       postmaster  postgres               [.] hash_search_with_hash_value
  0.41%           :13324  [kernel.kallsyms]      [k] compaction_alloc
  0.40%           :13561  [kernel.kallsyms]      [k] compaction_alloc

The compaction_alloc function points at a problem. You can turn off THP with:

echo never > /sys/kernel/mm/redhat_transparent_hugepage/enabled

Postgres versions before 9.4 do not specifically ask for huge pages, but it can be forced on them with always.

Here's a link to RedHat discouraging THP for database workloads.

StackExchange Database Administrators Q#98087, answer score: 4