Move unused buffers to freelist

As discussed and concluded in mail thread
(http://www.postgresql.org/message-id/006f01ce34f0$d6fa8220$84ef8660$@kapila
@huawei.com), for moving unused buffer's to freelist end,

I having implemented the idea and taken some performance data.

In the attached patch, bgwriter/checkpointer moves unused (usage_count =0 &&
refcount = 0) buffer's to end of freelist. I have implemented a new API
StrategyMoveBufferToFreeListEnd() to

move buffer's to end of freelist.

Performance Data :

Configuration Details

O/S - Suse-11

RAM - 24GB

Number of Cores - 8

Server Conf - checkpoint_segments = 256; checkpoint_timeout = 25 min,
synchronous_commit = 0FF, shared_buffers = 5GB

Pgbench - Select-only

Scalefactor - 1200

Time - Each run is of 20 mins

Below data is for average 3 runs of 20 minutes

8C-8T 16C-16T 32C-32T
64C-64T

HEAD 11997 8455 4989
2757

After Patch 19807 13296 8388
2821

Detailed each run data is attached with mail.

This is just the initial data, I will collect more data based on different
configuration of shared buffers and other configurations.

Feedback/Suggesions?

With Regards,

Amit Kapila.