Re: low priority postmaster threads?

I wrote:
> Chris Storah <cstorah@emis-support.demon.co.uk> writes:
>> What I am looking for is a postgres system that runs 100 users or so at
>> 'full speed', and major day long queries at a 'when idle' priority.

> The trouble here is that CPU nice doesn't (on most platforms) change the
> behavior of the I/O scheduler, so this would only be of use to the
> extent that your queries are CPU bound and not I/O bound.

Now that I think twice, there's an even more severe problem with trying
to nice() down an individual backend, namely priority inversion.

What happens when the low-priority process holds some lock or other,
and then a higher-priority process comes along and wants the lock?
The high-priority process has to wait, that's what.  But there's no
mechanism to raise the priority of the lower-priority lock holder, which
means that the high-priority process is now effectively lowered to the
lower priority; it may have to wait quite a long time, if there are
other high-priority processes sucking CPU away from the low-priority
guy.

In short, forget about nice'ing an individual backend; you probably
won't like the results.  Sorry.
        regards, tom lane