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

On Sat, Sep 24, 2016 at 9:07 AM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> 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.

No, it doesn't. Shared memory queues are single-reader, single-writer.

--
Robert Haas
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