Auto-tuning work_mem and maintenance_work_mem

Josh Berkus suggested here that work_mem and maintenance_work_mem could
be auto-tuned like effective_cache_size:

http://www.postgresql.org/message-id/50ECCF93.3060101@agliodbs.com

The attached patch implements this, closely matching the default values
for the default shared_buffers value:

test=> SHOW shared_buffers;
shared_buffers
----------------
128MB
(1 row)

test=> SHOW work_mem;
work_mem
----------
1310kB
(1 row)

test=> SHOW maintenance_work_mem;
maintenance_work_mem
----------------------
20971kB
(1 row)

Previous defaults were 1MB and 16MB, but the new defaults don't match
exactly because our max_connections is a power of 10 (100), not a power
of 2 (128). Of course, if shared_buffer is 10x larger, those defaults
become 10x larger.

FYI, I based maintenance_work_mem's default on shared_buffers, not on
work_mem because it was too complex to change maintenance_work_mem when
someone changes work_mem.

I will work on auto-tuning temp_buffers next. Any other suggestions?
wal_buffers is already auto-tuned.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +