Re: Trouble with hashagg spill I/O pattern and costing

On Tue, 2020-05-19 at 19:53 +0200, Tomas Vondra wrote:
> 
> And if there a way to pre-allocate larger chunks? Presumably we could
> assign the blocks to tape in larger chunks (e.g. 128kB, i.e. 16 x
> 8kB)
> instead of just single block. I haven't seen anything like that in
> tape.c, though ...

It turned out to be simple (at least a POC) so I threw together a
patch. I just added a 32-element array of block numbers to each tape.
When we need a new block, we retrieve a block number from that array;
or if it's empty, we fill it by calling ltsGetFreeBlock() 32 times.

I reproduced the problem on a smaller scale (330M groups, ~30GB of
memory on a 16GB box). Work_mem=64MB. The query is a simple distinct.

Unpatched master:
   Sort: 250s
   HashAgg: 310s
Patched master:
   Sort: 245s
   HashAgg: 262s

That's a nice improvement for such a simple patch. We can tweak the
number of blocks to preallocate, or do other things like double from a
small number up to a maximum. Also, a proper patch would probably
release the blocks back as free when the tape was rewound.

As long as the number of block numbers to preallocate is not too large,
I don't think we need to change the API. It seems fine for sort to do
the same thing, even though there's not any benefit.

Regards,
    Jeff Davis