Re: pg_dump performance

[Heikki Linnakangas <heikki(at)enterprisedb(dot)com>]

Jared Mauch wrote:
> On Wed, Dec 26, 2007 at 10:52:08PM +0200, Heikki Linnakangas wrote:
> > Jared Mauch wrote:
> > > 	pg_dump is utilizing about 13% of the cpu and the
> > > corresponding postgres backend is at 100% cpu time.
> > > (multi-core, multi-cpu, lotsa ram, super-fast disk).
> > > ...
> > > 	Any tips on getting pg_dump (actually the backend) to perform much closer 
> > > to 500k/sec or more?  This would also aide me when I upgrade pg versions 
> > > and need to dump/restore with minimal downtime (as the data never stops 
> > > coming.. whee).
> > I would suggest running oprofile to see where the time is spent. There 
> > might be some simple optimizations that you could do at the source level 
> > that would help.
> >
> > Where the time is spent depends a lot on the schema and data. For example, 
> > I profiled a pg_dump run on a benchmark database a while ago, and found 
> > that most of the time was spent in sprintf, formatting timestamp columns. 
> > If you have a lot of timestamp columns that might be the bottleneck for you 
> > as well, or something else.
> >
> > Or if you can post the schema for the table you're dumping, maybe we can  
> > make a more educated guess.
>
> 	here's the template table that they're all copies
> of:
>
> CREATE TABLE template_flowdatas (
>     routerip inet,
>     starttime integer,
>     srcip inet,
>     dstip inet,
>     srcifc smallint,
>     dstifc smallint,
>     srcasn integer,
>     dstasn integer,
>     proto smallint,
>     srcport integer,
>     dstport integer,
>     flowlen integer,
>     tcpflags smallint,
>     tosbit smallint
> );

I run a quick oprofile run on my laptop, with a table like that, filled 
with dummy data. It looks like indeed ~30% of the CPU time is spent in 
sprintf, to convert the integers and inets to string format. I think you 
could speed that up by replacing the sprintf calls in int2, int4 and 
inet output functions with faster, customized functions. We don't need 
all the bells and whistles of sprintf, which gives the opportunity to 
optimize.


A binary mode dump should go a lot faster, because it doesn't need to do 
those conversions, but binary dumps are not guaranteed to work across 
versions.

BTW, the profiling I did earlier led me to think this should be 
optimized in the compiler. I started a thread about that on the gcc 
mailing list but got busy with other stuff and didn't follow through 
that idea: http://gcc.gnu.org/ml/gcc/2007-10/msg00073.html

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com