Re: init_sequence spill to hash table

On 14.11.2013 14:38, David Rowley wrote:
> I've just completed some more benchmarking of this. I didn't try dropping
> the threshold down to 2 or 0 but I did tests at the cut over point and
> really don't see much difference in performance between the list at 32 and
> the hashtable at 33 sequences. The hash table version excels in the 16000
> sequence test in comparison to the unpatched version.
>
> Times are in milliseconds of the time it took to call currval() 100000
> times for 1 sequence.
> Patched Unpatched increased by 1 in cache 1856.452 1844.11 -1% 32 in
> cache 1841.84 1802.433 -2% 33 in cache 1861.558 not tested N/A 16000 in
> cache 1963.711 10329.22 426%

If I understand those results correctly, the best case scenario with the
current code takes about 1800 ms. There's practically no difference with
N <= 32, where N is the number of sequences touched. The hash table
method also takes about 1800 ms when N=33. The performance of the hash
table is O(1), so presumably we can extrapolate from that that it's the
same for any N.

I think that means that we should just completely replace the list with
the hash table. The difference with a small N is lost in noise, so
there's no point in keeping the list as a fast path for small N. That'll
make the patch somewhat simpler.
- Heikki