Postgres latency issues during memory compaction on EC2 (Ubuntu 14.04)
As dezso mentioned in the question comments, this did seem to be an issue with (possibly more recent versions of the) 3.13 kernel in Ubuntu Trusty - we switched to the Xenial HWE 4.4 kernel in Trusty and the problem seems to have gone away and compaction stalls are now very small and don't interfere.