[Mpi-22] Memory footprint concern
alexander.supalov at [hidden]
Thu May 8 16:52:34 CDT 2008
By the looks of it, MPI-3 is going to be big. Petascale machines may not
have OS we're accustomed to, dynamic libraries, and some other things.
Smaller system libraries - and smaller MPI - may be needed there. Some
of the envisioned MPI-3 features will be needed for some applications,
some won't. Same with MPI-2 and MPI-1. Defining subsets may help to open
a way to custom cut MPI libraries suitable for particular application
classes. How subsets will be implemented is a different matter.
[mailto:mpi-22-bounces_at_[hidden]] On Behalf Of Richard
Sent: Thursday, May 08, 2008 11:23 PM
To: MPI 2.2
Subject: [Mpi-22] Memory footprint concern
Can somebody help me understand this "smaller memory footprint" issue
that is part f the subsetting goal better. What systems does it affect?
What does "memory footprint" really mean? In the 64 bit address space,
virtual address range is not a problem.
On systems I am most familiar with (AIX and I have been told Linux too),
if you have a library that contains 1000 subroutines and you run a
program than only calls 6 then only the pages that are touched by code
for those 6 functions must get placed in real memory. The rest of the
object code stays on disk. Program and library text is demand paged. The
loading is on page boundries, not subroutine boundries.
With a shared library, if I run a program on a node and touch 6
subroutines and you run a different program that touches those 6 and 10
more then code for all 16 subroutines may be kept in memory but the rest
of the library will stay on disk. You and I will share the object code
for the 6 subroutines we are both calling.
Someone who wanted to make a libmpi that has MPI-1sided or MPI-IO well
isolated in the library structure so simple MPI jobs would not force
this extra code into memory could do that today. The user does not need
to promise not to call MPI-IO subroutines for them not to to take real
memory. The "subsets" would need to be devised by the MPI implementor
but would be transparent to the MPI user and not dictated by the
standard. The "subsets" the user did not call would remain paged out.
Perhaps all static data defined by the library will come into real
memory for each process but is there much reduction from being able to
somehow not bring in the static data MPI-IO would require because
somebody had promised not to use it?
Dick Treumann - MPI Team/TCEM
IBM Systems & Technology Group
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