Re: [WIP] speeding up GIN build with parallel workers

On Wed, Mar 16, 2016 at 2:55 PM, Constantin S. Pan <kvapen(at)gmail(dot)com> wrote:
>
> On Wed, 16 Mar 2016 12:14:51 +0530
> Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> > On Wed, Mar 16, 2016 at 5:41 AM, Constantin S. Pan <kvapen(at)gmail(dot)com>
> > wrote:
> > > 3. Tested on some real data (GIN index on email message body
> > > tsvectors). Here are the timings for different values of
> > > 'gin_shared_mem' and 'gin_parallel_workers' on a 4-CPU
> > > machine. Seems 'gin_shared_mem' has no visible effect.
> > >
> > > wnum mem(MB) time(s)
> > > 0 16 247
> > > 1 16 256
> > >
> >
> >
> > It seems from you data that with 1 worker, you are always seeing
> > slowdown, have you investigated the reason of same?
> >
>
> That slowdown is expected. It slows down because with 1 worker it
> has to transfer the results from the worker to the backend.
>
> The backend just waits for the results from the workers and merges them
> (in case wnum > 0).

Why backend just waits, why can't it does the same work as any worker
does? In general, for other parallelism features the backend also behaves
the same way as worker in producing the results if the results from workers
is not available.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com