Re: Admission Control

On Sat, Jun 26, 2010 at 11:03 AM, Martijn van Oosterhout
<kleptog(at)svana(dot)org> wrote:
> On Fri, Jun 25, 2010 at 03:15:59PM -0400, Robert Haas wrote:
>>  A
>> refinement might be to try to consider an inferior plan that uses less
>> memory when the system is tight on memory, rather than waiting.  But
>> you'd have to be careful about that, because waiting might be better
>> (it's worth waiting 15 s if it means the execution time will decrease
>> by > 15 s).
>
> I think you could go a long way by doing something much simpler. We
> already generate multiple plans and compare costs, why not just include
> memory usage as a cost? If you start doing accounting for memory across
> the cluster you can assign a "cost" to memory. When there are only a
> few processes running it's cheap and you get plans like now. But as the
> total memory usage increases you increase the "cost" of memory and
> there will be increased pressure to produce lower memory usage plans.
>
> I think this is better than just cutting plans out at a certain
> threshold since it would allow plans that *need* memory to work
> efficiently will still be able to.

That's an interesting idea.

> (It doesn't help in situations where you can't accurately predict
> memory usage, like hash tables.)

Not sure what you mean by this part. We already predict how much
memory a hash table will use.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company