[Mpi-22] Memory footprint concern

Richard Treumann treumann at [hidden]
Thu May 8 20:11:43 CDT 2008

Hi Alexander

If we would envision defining subsets in such a way that a machine  with
limited memory per node and un-fancy OS can clearly document what its MPI
offers, maybe it is not a huge addition to complexity. For example if an
MPI implementation could say it offers subsets:
MPI 1.2
MPI-Intercomm Collectives
and nothing else and the meaning would be well defined that may not be too

If you are expecting such a machine to have all or most of MPI-3 available
and each user would select only the parts he wants at job launch time it
seems to me the complexity for implementation and testing will be
prohibitive.  In particular if you hope subsets will give better
performance  because of what they leave out and really be compact because
they leave out most unneeded bytes of code the number of permutations seem
to me to make that a vain hope.

I have been ambivalent about whether my assertions proposal really has much
connection to the subsets concept mostly because I think to be useful
assertions must be fairly simple. The only assertion I am 100% sure will
pay off and be used is MPI_NO_EAGER_THROTTLE. There are MPI implementations
today that act as if the assertion MPI_NO_EAGER_THROTTLE were present on
all applications but that is not an option for a vendor MPI because if
somebody contacts service and says "Your MPI violates the standard" a
vendor like IBM must "fix" it.  IBM MPI supports MPI_CANCEL on MPI_ISEND
but at least one MPI does not because it is "too expensive".  In effect
they act like every application has the assertion MPI_NO_SEND_CANCELS. IBM
MPI could make use of MPI_NO_REQUEST_MIX if it was available.


Dick Treumann  -  MPI Team/TCEM
IBM Systems & Technology Group
Dept 0lva / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846         Fax (845) 433-8363

             <alexander.supalo                                          To 
             v_at_[hidden]>              "MPI 2.2"                           
             Sent by:                  <mpi-22_at_[hidden]>        
             mpi-22-bounces_at_li                                          cc 
                                       Re: [Mpi-22] Memory footprint       
             05/08/2008 05:52          concern                             
             Please respond to                                             
                 "MPI 2.2"                                                 

Dear Dick,

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.

Best regards.


From: mpi-22-bounces_at_[hidden]
[mailto:mpi-22-bounces_at_[hidden]] On Behalf Of Richard Treumann
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
Dept 0lva / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846 Fax (845) 433-8363

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