Re: Clock sweep not caching enough B-Tree leaf pages?

* Greg Stark (stark(at)mit(dot)edu) wrote:
> On Thu, Apr 17, 2014 at 12:21 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> > Ehhh. No. If it's a hot page that we've been holding in *our* cache
> > long enough, the kernel will happily evict it as 'cold' from *its*
> > cache, leading to...
>
> This is a whole nother problem.
>
> It is worrisome that we could be benchmarking the page replacement
> algorithm in Postgres and choose a page replacement algorithm that
> chooses pages that performs well because it tends to evict pages that
> are in the OS cache. And then one day (hopefully not too far off)
> we'll fix the double buffering problem and end up with a strange
> choice of page replacement algorithm.

That's certainly possible but I don't see the double buffering problem
going away any time particularly soon and, even if it does, it's likely
to either a) mean we're just using the kernel's cache (eg: something w/
mmap, etc), or b) will involve so many other changes that this will end
up getting changed anyway. In any case, while I think we should
document any such cache management system we employ as having this risk,
I don't think we should worry about it terribly much.

> It also means that every benchmark is super sensitive to the how large
> a fraction of system memory Postgres is managing. If A benchmark of a
> page replacement algorithm with 3GB shared buffers might perform well
> compared to others on a system with 8GB or 32GB total RAM but actually
> be choosing pages very poorly in normal terms and perform terribly on
> a system with 4GB total ram.

I'm not following you here- benchmarks are already sensitive to how much
of the system's memory PG is managing (and how much ends up being
*dedicated* to PG's cache and therefore unavailable for other work).

> Ideally what I would like to see is instrumentation of Postgres's
> buffer pinning so we can generate various test loads and then just run
> the different algorithms on them and measure precisely how many page
> evictions it's causing and when how often it's choosing pages that
> need to be read in soon after and so on. We shouldn't have to run
> Postgres to get these counts at all, just run the algorithm as we read
> through a text file (or database table) listing the pages being
> accessed.

Go for it. I'd love to see that also.

Thanks,

Stephen