BUG #14831: Intermittent write blocks

The following bug has been logged on the website:

Bug reference: 14831
Logged by: Harisankar Valiyattil
Email address: harisankar(dot)valiyattil(at)capitalone(dot)com
PostgreSQL version: 9.4.7
Operating system: linux
Description:

We use postgres on amazon RDS. We have a table with close to 15 million data
now and an application is constantly firing some update(insert/update)
operations to this table at around 25 transactions per second. the table has
a jsonb column which has a gin index on it; the jonb data has appx 20-40
attrinbutes in an average. The application keep on updating data on this
jsonb column.

While the traffic is running, we see that there is a sudden drop in the
write operations (no write happening in DB) causing increase in RDS cpu,
queue depth etc. after a few seconds (30 sec, 60 sec) things come back to
normal. If the traffic is constant this repeats at constant intervals.

Our work_mem is 4MB.

What could be causing this? What kind of troubleshooting we can do? What is
the process that runs in the background that causes this intermittant write
delay?