Should we increase the default vacuum_cost_limit?

Hi,

I've had to do quite a bit of performance investigation work this year
and it seems that I only too often discover that the same problem is
repeating itself... A vacuum_cost_limit that is still set to the 200
default value along with all 3 auto-vacuum workers being flat chat
trying and failing to keep up with the demand.

I understand we often keep the default config aimed at low-end
servers, but I don't believe we should categorise this option the same
way as we do with shared_buffers and work_mem. What's to say that
having an auto-vacuum that runs too slowly is better than one that
runs too quickly?

I have in mind that performance problems arising from having
auto-vacuum run too quickly might be easier to diagnose and fix than
the ones that arise from it running too slowly. Certainly, the
aftermath cleanup involved with it running too slowly is quite a bit
more tricky to solve.

Ideally, we'd have something smarter than the cost limits we have
today, something that perhaps is adaptive and can make more use of an
idle server than we do now, but that sounds like a pretty large
project to consider having it working this late in the cycle.

In the meantime, should we consider not having vacuum_cost_limit set
so low by default?

I have in mind something in the ballpark of a 5x to 10x increase. It
seems the standard settings only allow for a maximum of ~3.9MB/s dirty
rate and ~7.8MB/s shared buffer miss rate. That seems pretty slow
even for the micro SD card that's in my 4-year-old phone. I think we
should be aiming for setting this to something good for the slightly
better than average case of modern hardware.

The current default vacuum_cost_limit of 200 seems to be 15 years old
and was added in f425b605f4e.

Any supporters for raising the default?

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services