Subject: Re: Bad response...
To: Eric Haszlakiewicz <>
From: Michael van Elst <>
List: current-users
Date: 08/31/2004 07:38:38

> 	It _isn't_ a problem for the system to be writing stuff out to disk
> while your interactive program is running.  Interactive response of a
> _in-memory_ program will be more dependant on CPU availability, which I/O
> operations shouldn't take a whole lot of.

that's correct. But we are not talking about I/O but about memory pages
that are locked up for some time and the fact that "_in-memory_" is
rarely seen with todays huge applications.

We are also talking about the fact that disk writing programs fill
the filecache with data that is hardly reused for the only benefit
that the program appears to finish early (assuming that there is
enough filecache for all data it writes). If you write more data than
fits into your filecache it will just thrash and is absolutely

A limit to the dirty pages in the filecache will help for all these

On the other hand the limit slows down workloads that alternate between
large disk-writes and computations, i.e. your typical system build
that utilizes the filecache is a disk write buffer. But I believe
the effect won't be that large with a reasonable limit (~ 10MByte).

Now, asking on how to determine that limit is an interesting question.
But a simple constant (aka 'sysctl knob') is probably good enough.

                                Michael van Elst
                                "A potential Snark may lurk in every tree."