Reducing bgwriter wakeups

Recent changes for power reduction mean that we now issue a wakeup
call to the bgwriter every time we set a hint bit.

However cheap that is, its still overkill.

My proposal is that we wakeup the bgwriter whenever a backend is
forced to write a dirty buffer, a job the bgwriter should have been
doing.

This significantly reduces the number of wakeup calls and allows the
bgwriter to stay asleep even when very light traffic happens, which is
good because the bgwriter is often the last process to sleep.

Seems useful to have an explicit discussion on this point, especially
in view of recent performance results.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
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