| From: | Peter Geoghegan <pg(at)heroku(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz>, Jeff Davis <pgsql(at)j-davis(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com>, Amit Langote <amitlangote09(at)gmail(dot)com>, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, Fabrízio Mello <fabriziomello(at)gmail(dot)com>, Thom Brown <thom(at)linux(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Parallel Seq Scan |
| Date: | 2015-09-16 22:56:09 |
| Message-ID: | CAM3SWZTZnhu0awWe6QO8xN2kw_rKS1NpdpxRfaES3LJ_MnEMfA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Jul 22, 2015 at 10:44 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> One thing I noticed that is a bit dismaying is that we don't get a lot
> of benefit from having more workers. Look at the 0.1 data. At 2
> workers, if we scaled perfectly, we would be 3x faster (since the
> master can do work too), but we are actually 2.4x faster. Each
> process is on the average 80% efficient. That's respectable. At 4
> workers, we would be 5x faster with perfect scaling; here we are 3.5x
> faster. So the third and fourth worker were about 50% efficient.
> Hmm, not as good. But then going up to 8 workers bought us basically
> nothing.
...sorry for bumping up this mail from July...
I don't think you meant to imply it, but why should we be able to
scale perfectly? Even when the table fits entirely in shared_buffers,
I would expect memory bandwidth to become the bottleneck before a
large number of workers are added. Context switching might also be
problematic.
I have almost no sense of whether this is below or above par, which is
what I'm really curious about. FWIW, I think that parallel sort will
scale somewhat better.
--
Peter Geoghegan
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