Re: Seq scans roadmap

On Tue, 2007-05-08 at 07:47 -0400, Luke Lonergan wrote:
> Heikki,
>
> On 3A: In practice, the popular modern OS'es (BSD/Linux/Solaris/etc)
> implement dynamic I/O caching. The experiments have shown that benefit
> of re-using PG buffer cache on large sequential scans is vanishingly
> small when the buffer cache size is small compared to the system memory.
> Since this is a normal and recommended situation (OS I/O cache is
> auto-tuning and easy to administer, etc), IMO the effort to optimize
> buffer cache reuse for seq scans > 1 x buffer cache size is not
> worthwhile.
>

I think 3A is still an interesting idea, but I agree that it needs some
results to back it up. Let's save 3A for the next release so that we
have more time to see.

> To that point - an important factor in achieving max I/O rate for large
> tables (> 1 x bufcache) is avoiding the pollution of the CPU L2 cache.
> This is commonly in the range of 512KB -> 2MB, which is only important
> when considering a bound on the size of the ring buffer. The effect has
> been demonstrated to be significant - in the 20%+ range. Another thing
> to consider is the use of readahead inside the heapscan, in which case
> sizes >= 32KB are very effective.

One thing I'd like us to keep in mind is to have a reasonable number of
buffers active for a sequential scan. If the number is too small, my
sync scans might not even work. Right now my patch only reports every 16
pages, so 32KB (=4 pages) is not going to work for sync scans (I suppose
only testing will tell).

Regards,
Jeff Davis