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

On Mon, Sep 26, 2016 at 3:40 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> On Mon, Sep 26, 2016 at 6:58 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> That requires some kind of mutual exclusion mechanism, like an LWLock.
>>
>> No, it doesn't. Shared memory queues are single-reader, single-writer.
>
> The point is that there is a natural dependency when merging is
> performed eagerly within the leader. One thing needs to be in lockstep
> with the others. That's all.

I don't know what any of that means. You said we need something like
an LWLock, but I think we don't. The workers just write the results
of their own final merges into shm_mqs. The leader can read from any
given shm_mq until no more tuples can be read without blocking, just
like nodeGather.c does, or at least it can do that unless its own
queue fills up first. No mutual exclusion mechanism is required for
any of that, as far as I can see - not an LWLock, and not anything
similar.

--
Robert Haas
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