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Re: Resource management in 7.4

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Gavin Sherry <swm(at)linuxworld(dot)com(dot)au>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Resource management in 7.4
Date: 2002-12-21 05:20:56
Message-ID: 27931.1040448056@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Gavin Sherry <swm(at)linuxworld(dot)com(dot)au> writes:
> I've been thinking about resource management and postgres. I want to
> develop a user profile system (a-la oracle) which allows a DBA to
> restrict/configure access to system resources. This would allow a DBA to
> configure how much CPU time can be used per query/session for any user,
> the number blocks that can be read/written by a user per query, and
> perhaps some other things (see below).

I've got really serious reservations about this whole idea.  I don't
like expending even one CPU cycle on it, and I don't like introducing a
potential cause of unnecessary query failure, and I don't believe that
the average DBA would be capable of configuring it intelligently.

To point out just one problem: in the current system design, the backend
that actually issues a write request is not necessarily, or even
probably, the one that dirtied the page.  And you can NOT refuse to
write a dirtied page because of some misbegotten notion about resource
limits; system reliability will go to zero if you do.

Another example is that the cost of verifying transaction completion is
actually paid by the first transaction to visit a tuple after the
tuple's authoring transaction completes.  Should a transaction be
penalized if it's foolish enough to do a seqscan shortly after someone
else does a mass insert or update?

In general, I think that per-user resource management policies would
force us to adopt inefficient algorithms that don't share overhead costs
across the whole community.  I'm not eager for that...

			regards, tom lane

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