Re: Parallel tuplesort (for parallel B-Tree index creation)

On Thu, Sep 22, 2016 at 8:57 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Sep 22, 2016 at 3:51 PM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>> It'd be good if you could overlap the final merges in the workers with the
>> merge in the leader. ISTM it would be quite straightforward to replace the
>> final tape of each worker with a shared memory queue, so that the leader
>> could start merging and returning tuples as soon as it gets the first tuple
>> from each worker. Instead of having to wait for all the workers to complete
>> first.
>
> If you do that, make sure to have the leader read multiple tuples at a
> time from each worker whenever possible. It makes a huge difference
> to performance. See bc7fcab5e36b9597857fa7e3fa6d9ba54aaea167.

That requires some kind of mutual exclusion mechanism, like an LWLock.
It's possible that merging everything lazily is actually the faster
approach, given this, and given the likely bottleneck on I/O at htis
stage. It's also certainly simpler to not overlap things. This is
something I've read about before [1], with "eager evaluation" sorting
not necessarily coming out ahead IIRC.

[1] http://digitalcommons.ohsu.edu/cgi/viewcontent.cgi?article=1193&context=csetech
--
Peter Geoghegan