Re: Support Parallel Query Execution in Executor

"Jonah H. Harris" <jonah(dot)harris(at)gmail(dot)com> writes:
> On 4/8/06, Markus Schiltknecht <markus(at)bluegap(dot)ch> wrote:
>> Ehm.. which additional I/O cost? Or do you count inter-process
>> communication to I/O?

> Inter-process will add a minimal amount, but if it's done correctly,
> you'll still end up ahead.

This is exactly the bit of optimism I was questioning. We've already
been sweating blood trying to reduce multiprocessor contention on data
structures in which collisions ought to be avoidable (ie, buffer arrays
where you hope not everyone is hitting the same buffer at once). I
think passing large volumes of data between different processes is going
to incur quite a lot of locking overhead, pipeline stalls for cache line
transfers, etc, etc, because heavy contention for the transfer buffer is
simply not going to be avoidable.

regards, tom lane