Re: init_sequence spill to hash table

On Fri, Nov 15, 2013 at 3:03 AM, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com
> wrote:

> 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
>

I had thought that maybe the biggest type of workloads might only touch 1
or 2 sequences, though it may be small but I had thought there would be an
overhead in both cycles and memory usage in creating a hash table for these
light usages of sequence backends. It would certainly make the patch more
simple by removing this and it would also mean that I could remove the
sometimes unused ->next member from the SeqTableData struct which is just
now set to NULL when in hash table mode. If you think it's the way to go
then I can make the change, though maybe I'll hold off the refactor for now
as it looks like other ideas have come up around rel cache.

Regards

David Rowley