On Thu, Jun 30, 2016 at 7:08 PM, Andres Freund <and...@anarazel.de> wrote: > If you have a big enough index (maybe ~150GB+), sure. Before that, > probably not. > > It's usually pretty easy to see in cpu profiles whether this issue > exists.
I think that this is a contributing factor to why merging in parallel CREATE INDEX becomes much more CPU bound when building such very large indexes, which Corey Huinker has benchmarked using an advanced copy of the patch. He has shown cases that are sped up by 3.6x when 8 parallel workers are used (compared to a serial CREATE INDEX), but a several hundred gigabyte index case only sees a speedup of about 1.5x. (This bottleneck affects serial CREATE INDEX merging just as much as parallel, since that part isn't parallelized, but it's far more noticeable with parallel CREATE INDEX simply because merging in the leader becomes a huge bottleneck). Those two cases were not exactly comparable in perhaps several other ways, but even still my sense is that that this can be at least partially explained by md.c bottlenecks. This is something that we'll need to confirm through profiling. Hopefully it's just this one bottleneck. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers