On Wed, May 7, 2014 at 2:58 PM, Peter Geoghegan <pg@heroku.com> wrote:
> On Wed, May 7, 2014 at 11:50 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>> But that does not mean, as the phrase "folk
>> wisdom" might be taken to imply, that we don't know anything at all
>> about what actually works well in practice.
>
> Folk wisdom doesn't imply that. It implies that we think this works,
> and we may well be right, but there isn't all that much rigor behind
> some of it. I'm not blaming anyone for this state of affairs. I've
> heard plenty of people repeat the "don't exceed 8GB" rule - I
> regularly repeated it myself. I cannot find any rigorous defense of
> this, though. If you're aware of one, please point it out to me.
I'm not sure the level of rigor you'd like to see is going to be
available here. Complex systems have complex behavior; that's life.
At any rate, I'm not aware of any rigorous defense of the "don't
exceed 8GB" rule. But, #1, I'd never put it that simply. What I've
found is more like this: If it's possible to size shared_buffers so
that the working set fits entirely within shared_buffers, that
configuration is worthy of strong consideration. Otherwise, you
probably want to keep shared_buffers low in order to avoid
checkpoint-related I/O spikes and minimize double buffering; try 25%
of system memory up to 512MB on Windows or up to 2GB on 32-bit Linux
or up to 8GB on 64-bit Linux for starters, and then tune based on your
workload.
And #2, I think the origin of the 8GB number on 64-bit non-Windows
systems is that people found that checkpoint-related I/O spikes became
intolerable when you went too much above that number. On some
systems, the threshold is lower than that - for example, I believe
Merlin and others have reported numbers more like 2GB than 8GB - and
on other systems, the threshold is higher - indeed, some people go way
higher and never hit it at all. I agree that it would be nice to
better-characterize why different users hit it at different levels,
but it's probably highly dependent on hardware, workload, and kernel
version, so I tend to doubt it can be characterized very simply.
If I had go to guess, I'd bet that fixing Linux's abominable behavior
around the fsync() call would probably go a long way toward making
higher values of shared_buffers more practical.
--
Robert Haas
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