Re: PG_DUMP very slow because of STDOUT ??

Andras Fabian wrote:
> So the kernel function it is always idling on seems to be congestion_wait ...

Ugh, not that thing again.  See
http://www.westnet.com/~gsmith/content/linux-pdflush.htm ; that chunk of
code has cost me weeks worth of "why isn't the kernel writing things the
way I asked it?" trouble in the past.  I know the kernel developers have
been fiddling with pdflush again recently, they might have introduced
yet another bug into how it handles heavy write volume.  You can reduce
dirty_ratio and dirty_background_ratio to try and improve things, but
the congestion code will thwart any attempt to make them really low.

You might monitor what shows up as "Dirty:" in /proc/meminfo to see if
that lines up with the slow periods; example of what bad output looks
like at
http://notemagnet.blogspot.com/2008/08/linux-write-cache-mystery.html

--
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us