BufferSync and bgwriter

The idea that bgwriter smooths out the response time of transactions is
only true if the buffer lists T1 and T2 have *some* clean buffers
available for use when performing I/O. The alternative is that
transactions unlucky enough to encounter the no-clean-buffers situation
have to clean a space for themselves, effectively making the bgwriter
redundant.

In BufferSync, we start off by calling StrategyDirtyBufferList to make a
list of all the dirty buffers. Even though we know we are limited to
maxpages, we still scan the whole of shared_buffers (...making it a very
expensive call and thereby causing us to increase bgwriter_delay, which
then negates the cleaning effect as described above).

Once we've got the list, we limit ourselves to only using maxpages of
the list that we just built. We do it that way round to allow
bgwriter_percent to calculate how many of the dirty buffers it should
flush, on the assumption that percent < 100.

If the bgwriter_percent = 100, then we should actually do the sensible
thing and prepare the list that we need, i.e. limit
StrategyDirtyBufferList to finding at most bgwriter_maxpages.

Thus if you have a large shared_buffers, you can still have a relatively
frequent bgwriter_delay, so that the bgwriter can keep the LRUs of the
T1 and T2 lists free for use...and so let backends get on with useful
work.

Patch which implements this attached, for discussion.

Mark, any chance we could run this patch on STP to test whether it has a
beneficial performance effect? Re-run test 207 to compare?

I'll be asking for this in 8.0, if it works, for all the same
performance reasons discussed previously as well as coming under the
header of "bgwriter default changes" since this effects the default
behaviour when bgwriter_percent=100.

There are some other ideas for 8.1, but that can wait.

--
Best Regards, Simon Riggs