Re: Admission Control

Simon, Mark,

> Actually only 1 lock check per query, but certainly extra processing and
> data structures to maintain the pool information... so, yes certainly
> much more suitable for DW (AFAIK we never attempted to measure the
> additional overhead for non DW workload).

I recall testing it when the patch was submitted for 8.2., and the
overhead was substantial in the worst case ... like 30% for an in-memory
one-liner workload.

I've been going over the greenplum docs and it looks like the attempt
to ration work_mem was dropped. At this point, Greenplum 3.3 only
rations by # of concurrent queries and total cost. I know that work_mem
rationing was in the original plans; what made that unworkable?

My argument in general is that in the general case ... where you can't
count on a majority of long-running queries ... any kind of admission
control or resource management is a hard problem (if it weren't, Oracle
would have had it before 11). I think that we'll need to tackle it, but
I don't expect the first patches we make to be even remotely usable.
It's definitely not an SOC project.

I should write more about this.

--
-- Josh Berkus
PostgreSQL Experts Inc.
http://www.pgexperts.com