Re: Memory intensive processes

Richard B. Johnson (
Wed, 11 Dec 1996 08:57:59 -0500 (EST)

On Tue, 10 Dec 1996, William Burrow wrote:

> On Tue, 10 Dec 1996, Richard B. Johnson wrote:
> > Amongst other things, the VAX has a "modfied page writer". It works like
> > this. When a process is allocated memory, the initial memory comes from
> > a pool of shared zero-filled pages. These pages don't actually get
> > owned by a specific process until a process actually writes to one.
> VAX/VMS (and quite recently too). Has this been implemented in Linux?
> Is somebody planning to implement it? Was it you who wrote me about this
> before??? Deja vu on this.
I have written about this before. Too often I get interrupted with a
"work break" so I haven't mucked around with the kernel except to help
fix some occasional problems in drivers.

> Consider you have a process with a very large set of matrices. Most of
> these could be sparse (eg mostly zeros). The IEEE representation of
> floating point zeroes is all zeroes. Therefore, the scheme you mention
> could in fact suitably represent in a single page a large chunk of memory
> that would otherwise be wasted (filled with zeroes). This alone could get
> some of the large process blues off of Linux' back.
This is true. Also the act of READING zero-filled pages should not cause
a trap to the operating system. Many FFTs null-fill a lot of RAM and
certainly never write to it.

> > VAX/VMS has quotas on just about everything. The maximum working-set
> > size, i.e., the maximum virtual pages that a process can own, is
> > set via AUTHORIZE. Further, SYSGEN parameters also set sizes system-
> > wide.
> I once heard a joke that VMS would log when a user sneezed. Most
> Unixheads don't seem to like VMS all that much, though it had some
> very good ideas.

VAXen were designed in the days of slow, poor performing hardware. We
can learn a lot from the sucesses of VAX/VMS, but should not copy its
failures. Memory management is one area in which it excelled. Can you
imagine; 35 users compiling FORTRAN on a system with 4 megabytes of RAM?

Yes it worked. Of course there was a lot of help from some of the hardware.
The tty boards had seperate CPUs that handled all the escape-sequences, etc.
The CPU was never called upon to write pages of Screen memory a'la Xwin.

> {excellent ideas elided for brevity}
> > Now, what this does is help prevent a runaway task from taking all the
> > system resources. If your task is a memory hog, it gets slowed down
> > by this allocation strategy while other tasks end up using CPU time
> > stolen from the memory hog.
> So, that would be half an answer anyway. I don't see how the kernel can
> do much about how a process decides to access memory. Gnuchess is
> particularly bad on memory restricted systems (that guy with the 8meg RAM
> 386 ought to give it a shot to see what this problem is about). The days
> of assuming infinite, high speed memory are slowly moving away also, as
> CPUs ramp to faster clocks and depend more on cache memory.
I just got a patch from "b" which causes the swapper to sleep, i.e., give
the CPU to someone else after some hard paging. I will try it.