FAQ

On 28 June 2013 05:52, Amit Kapila wrote:


As per my understanding Summarization of points raised by you and Andres
which this patch should address to have a bigger win:

1. Bgwriter needs to be improved so that it can help in reducing usage
count
and finding next victim buffer
(run the clock sweep and add buffers to the free list).
2. SetLatch for bgwriter (wakeup bgwriter) when elements in freelist are
less.
3. Split the workdone globallock (Buffreelist) in StrategyGetBuffer
(a spinlock for the freelist, and an lwlock for the clock sweep).
4. Separate processes for writing dirty buffers and moving buffers to
freelist
5. Bgwriter needs to be more aggressive, logic based on which it calculates
how many buffers it needs to process needs to be improved.
6. There can be contention around buffer mapping locks, but we can focus on
it later
7. cacheline bouncing around the buffer header spinlocks, is there anything
we can do to reduce this?
My perspectives here would be

* BufFreelistLock is a huge issue. Finding a next victim block needs to be
an O(1) operation, yet it is currently much worse than that. Measuring
contention on that lock hides that problem, since having shared buffers
lock up for 100ms or more but only occasionally is a huge problem, even if
it doesn't occur frequently enough for the averaged contention to show as
an issue.

* I'm more interested in reducing response time spikes than in increasing
throughput. It's easy to overload a benchmark so we get better throughput
numbers, but that's not helpful if we make the system more bursty.

* bgwriter's effectiveness is not guaranteed. We have many clear cases
where it is useless. So the question should be to continually answer the
question: do we need a bgwriter and if so, what should it do? The fact we
have one already doesn't mean it should be given things to do. It is a
possible option that things may be better if it did nothing. (Not saying
that is true, just that we must consider that optione ach time).

--
  Simon Riggs http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services

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