>>>>> "M" == Mike A Harris <mharris@meteng.on.ca> writes:
M> Can someone please explain to me how and why an app would
M> allocate a large amount of memory and then not use it? Please
M> give specific examples since I can't fathom the reason. I think
M> an app that requests a lot of memory and then doesn't use it is
M> BAD - broken as designed. I can see legitimate allocation of
M> SMALL amounts of unused memory as speculation in time critical
M> code, or for optimization, etc.. but not large amounts.
2 examples off the top of my head:
1) fork/exec. The fork() effectively allocates again the VM of the
original process. The exec() may be a much smaller one. The (rather
extreme) example used in the IRIX newsgroups is the 100GB+
simulation running for a week that needs to popen() (ie: fork/exec)
a much smaller program (eg: ls) for some reason. You want to tell the
customer that they need to buy double the memory and/or swap when a
minor configuration change will make it work?
2) a lot of those large (FORTRAN) simulations allocate huge arrays,
then use only a small amount of it, depending on the specifics of
the problem that run. Yes, they could be recoded to use
sparse-matrix techniques, but that takes time and resources that
could be better used. And may actually run slower.
The kernel VM allocation system can't tell the difference between 1 & 2.
-- Scott Henry <scotth@sgi.com> / Help! My disclaimer is missing! IRIX MTS, / GIGO *really* means: Garbage in, Gospel Out Silicon Graphics, Inc / http://reality.sgi.com/scotth/- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
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