Re: checkpointer continuous flushing

On Wed, Jan 20, 2016 at 9:02 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> Chatting on IM with Heikki, I noticed that we're pretty pessimistic in
> SetHintBits(). Namely we don't set the bit if XLogNeedsFlush(commitLSN),
> because we can't easily set the LSN. But, it's actually fairly common
> that the pages LSN is already newer than the commitLSN - in which case
> we, afaics, just can go ahead and set the hint bit, no?
>
> So, instead of
> if (XLogNeedsFlush(commitLSN) && BufferIsPermanent(buffer)
> return; /* not flushed yet, so don't set hint */
> we do
> if (BufferIsPermanent(buffer) && XLogNeedsFlush(commitLSN)
> && BufferGetLSNAtomic(buffer) < commitLSN)
> return; /* not flushed yet, so don't set hint */
>
> In my tests with pgbench -s 100, 2GB of shared buffers, that's recovers
> a large portion of the hint writes that we currently skip.

Dang. That's a really good idea. Although I think you'd probably
better revise the comment, since it will otherwise be false.

--
Robert Haas
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