The ParGAP (Parallel GAP) package provides a way of writing parallel programs using the GAP language. Former names of the package were ParGAP/MPI and GAP/MPI; the word MPI refers to Message Passing Interface, a well-known standard for parallelism. ParGAP is based on the MPI standard, and this distribution includes a subset implementation of MPI, to provide a portable layer with a high level interface to BSD sockets. Since knowledge of MPI is not required for use of this software, we now refer to the package as simply ParGAP. For more information visit the author's ParGAP home page at: http://www.ccs.neu.edu/home/gene/pargap.html
For some background reading, see Coo95 and Coo97.
This first chapter is intended to help a new user set up ParGAP and run through some quick examples: see
RequirePackage); and
The later chapters present detailed explanations of the facilities of
ParGAP. Because parallel programming is sufficiently different from
sequential programming, this author recommends printing out at least
Chapters 1 through MasterSlave Tutorial, and skimming through those
chapters for areas of interest, before returning to the terminal to try
out some of the ideas. This document can be found in
.../pkg/pargap/doc/manual.dvi of the software distribution. You may
also want to print the index at the end of manual.dvi. In particular,
the heading example in the index, or ??example from within GAP,
should be useful. If you prefer postscript, the UNIX command dvips will
convert that file to postscript form.
The development of ParGAP was partially supported by National Science Foundation grants CCR-9509783 and CCR-9732330.
ParGAP is currently functional only on UNIX installations. (Cygwin for Windows is also an option, if you would like to port it.) ParGAP can be installed on top of an existing GAP installation. See Section Installing ParGAP for instructions on installation of ParGAP. At the time that ParGAP is invoked, a special ``procgroup'' file must be available to tell ParGAP which processors to use for slave processors. See sections Installing ParGAP and Extended Example for instructions on invoking ParGAP. If there are questions or bugs concerning ParGAP, please write to: gene@ccs.neu.edu
If one wishes only to try out the parallel features, the first five pages of this manual (through the section on the slave listener) will suffice for installation, and using it. For the more advanced user who wishes to design new parallel algorithms or port old sequential code to a parallel environment, it is strongly recommended to also read the sections following on from Section Basic Concepts for the TOP-C model (MasterSlave).
ParGAP should be invoked via the script bin/pargap.sh created by the
installation process which invokes GAP_ROOT_DIR/bin/ARCH/pargapmpi,
where ARCH depends on your system but is the same directory in which
the gap binary is found. MPI and the higher layers will not be
available if the binary is invoked in the standard way as gap. This is
a feature, since a single binary and source distribution serves both for
the standard GAP and for ParGAP.
ParGAP is implemented in three layers: 1) MPI, 2) Slave Listener, and 3) Master Slave (TOP-C abstraction). Most users will find that the two highest layers (Slave Listener and Master Slave) meet all their needs.
1) MPI:Error break in the presence of errors.
MPI_Init() (see MPI_Init) and MPI_Finalize()
(see MPI_Finalize) are invoked automatically by ParGAP.
MPI_tabMPI_Send; ) will cause it to display the calling
syntax. The same information is displayed after an incorrect call.
The return value is typically obvious. MPI is implemented in
src/pargap.c. The standard distribution uses a simple, subset
implementation of MPI in pkg/gapmpi/mpinu/, which is implemented on
top of a standard sockets interface. It is possible to substitute
other implementations of MPI.
2) Slave Listener:
*Msg*
e.g. SendMsg() (see SendMsg), RecvMsg() (see RecvMsg),
ProbeMsg() (see ProbeMsg). Since the slave is in a
receive-eval-send loop, every SendMsg(cmd) on the master must be
balanced by a later RecvMsg(). SendRecvMsg() (see SendRecvMsg)
is provided to combine these steps. A few parallel utilities are also
included, such as ParRead() (ParRead), ParList() (ParList),
ParEval() (ParEval), etc.
SendMsg() or ParEval() would be evaluated
locally before being sent across the network. For this reason,
arguments can also be given as strings, to delay evaluation until
reaching the destination process. Hence, real strings must be quoted:
ParEval("x:="abc";"); Additionally, multiple commands are valid,
and the final ``;'' of the string is optional. So, one can write:
BroadcastMsg("x:=\"abc\"; Print(Length(x), \"\\n\")");;
3) Master Slave:
1)
2)
Installing ParGAP should be relatively simple. However, since there are many interactions both with the GAP kernel and with the UNIX operating system, in a minority of cases, manual intervention will be necessary. If you are part of this minority, please see the section Problems with Installation. The most common problem is the local security policy; ParGAP is more pleasant to use when you don't have to manually provide the password for each slave. See section Problems with Passwords (Getting Around Security) for suggestions in this respect.
To install the ParGAP package, move the file pargap-XXX.zoo or
pargap-XXX.tar.gz (for some version number XXX of ParGAP) into
the pkg directory in which you plan to install ParGAP. Usually, this
will be the directory pkg in the hierarchy of your version of GAP 4
(in fact, currently it is not possible to have the pkg directory
separate from GAP's pkg directory; we hope to remedy this in future
versions of ParGAP so that it will also possible to keep an additional
pkg directory in your private directories; section Installing GAP Packages of the GAP 4 reference manual gives details on how to do this,
when it's possible.)
Now change into the pkg directory in which you plan to install
ParGAP. If you got a .zoo file, unpack it with:
unzoo -x pargap-XXX
If you got a .tar.gz file and your tar command supports the z
option, unpack it with:
tar zxf pargap-XXX.tar.gz
or otherwise unpack in two steps with:
gunzip pargap-XXX.tar
tar xvf pargap-XXX.tar
Whether you got the .zoo or .tar.gz archive you should now have a new
directory pargap. As for a generic GAP package, do:
cd pargap ./configure ../.. make
If your version of GAP is earlier than GAP 4.3 you will first need
to adjust GAP's lib/init.g file; see item 0. of Section Problems with Installation.
Your ParGAP should now be ready to use. Now read the next section
which decribes how to run ParGAP (if you are reading this from
GAP's on-line help, type: ?>).
After doing the configure and make steps of ParGAP's installation
process (see Section Installing ParGAP), you should find in ParGAP's
bin subdirectory a script
pargap.sh
which you should use to start ParGAP. (ParGAP can not be started
by starting GAP 4 in the usual way, and using RequirePackage; doing
so will result in Info-ed advice to read this section.) Edit the
pargap.sh script if necessary, copy it to a standard path and rename it
according to how you intend to call ParGAP (e.g. rename it: pargap).
Also, in the bin subdirectory is a sample procgroup file which
defines the master and slave processes that will be used by ParGAP.
When ParGAP is started it looks for a file called procgroup in the
current directory, unless the -p4pg option is used. Thus if you renamed
your shell script pargap, the following are valid ways of starting
ParGAP:
pargap
(if current directory contains the file: procgroup), or
pargap -p4pg myprocgroupfile
(where myprocgroupfile is the complete path of your procgroup file -- there is no restriction on how you name it).
If you had trouble installing ParGAP, see the section Problems with Installation. Otherwise continue onto Section Extended Example and try out ParGAP.
Note:
The script pargap.sh defines the program that runs ParGAP as
pargapmpi. In fact, after installation pargapmpi is a symbolic link
to the GAP binary named gap. The same binary runs both GAP and
ParGAP; when the binary is invoked as gap GAP runs in the usual
way without any parallel features; only when the binary is invoked as
pargapmpi are the parallel features incorporated. See
Section Modifying the GAP kernel for more details.
Now you are ready to test your installation, try the example in the
following section (if you are reading this from GAP's on-line help,
type: ?>).
After installation, try it out. Invoke ParGAP as described in
Section Running ParGAP and try the example below (but substitute your
own program where you see "/home/gene/myprogram.g"). The commands in
this first example are also found in the README file. So, you may wish
to copy text from the README file and paste it into a ParGAP session.
If you are using the unmodified procgroup file, your remote slaves
will be other processes on your local machine. It is a good idea to run
only on your local machine for your first experiments and while you are
debugging parallel programs. When you wish to experiment with using
remote machines, you can then proceed to the following section, Invoking ParGAP with Remote Slaves.
gap> # This assumes your procgroup file includes two slave processes.
gap> PingSlave(1); #a `true' response indicates Slave 1 is alive
true
gap> # Print() on slave appears on standard output
gap> # i.e. after the master's prompt.
gap> SendMsg( "Print(3+4)" );
gap> 7
gap> # A <return> was input above to get a fresh prompt.
gap> #
gap> # To get special characters (including newline: `\n')
gap> # into a string, escape them with a `\'.
gap> SendMsg( "Print(3+4,\"\\n\")" );
gap> 7
gap> # Again, a <return> was input above after the 7 and new-line
gap> # were printed to get a fresh prompt.
gap> #
gap> # Each SendMsg() is normally balanced by a RecvMsg().
gap> SendMsg( "3+4", 2);
gap> RecvMsg( 2 );
7
gap> # The following is equivalent to the two previous commands.
gap> SendRecvMsg( "3+4", 2);
7
gap> # Flush any messages that are pending. The response is
gap> # the number of messages flushed. (Above, the two
gap> # SendMsg("Print...") (to the default slave: 1) did not
gap> # have a corresponding RecvMsg() command.)
gap> FlushAllMsgs();
2
gap> # As with Print() the result of Exec() appears on standard
gap> # output. Print() and Exec() are each `no-value' functions,
gap> # and so the result of a RecvMsg() in these cases
gap> # is "<no_return_val>".
gap> SendRecvMsg( "Exec(\"pwd\")" ); # Your pwd will differ :-)
/home/gene
"<no_return_val>"
gap> # Put default slave into an infinite loop.
gap> SendMsg("while true do od");
gap> # Default slave can't execute the next command until it's
gap> # finished with the previous command.
gap> SendMsg("Print(\"WAKE UP\\n\")");
gap> # Check to see if a message is waiting to be collected but
gap> # return immediately (i.e. don't get blocked by waiting for
gap> # a message to appear). A `false' response indicates the
gap> # infinite loop hasn't terminated and produced a value yet!
gap> ProbeMsgNonBlocking();
false
gap> # Send an interrupt to each slave, slave 1 will see the
gap> # following command and print `WAKE UP', and then all
gap> # pending messages are flushed.
gap> ParReset();
... resetting ...
WAKE UP
0
gap> # The return value, 0, from ParReset() indicates there
gap> # were 0 pending messages flushed, confirming correctness
gap> # of ProbeMsgNonBlocking() when it returned "false"
gap> SendRecvMsg( "a:=45; 3+4", 1 );
7
gap> # Note "a" is defined on slave 1, not slave 2.
gap> SendMsg( "a", 2 ); # Slave prints error, output on master
gap> Variable: 'a' must have a value
gap> # <return> entered to get fresh prompt.
gap> RecvMsg( 2 ); # No value for last SendMsg() command
"<no_return_val>"
gap> RecvMsg( 1 );
45
gap> myfnc := function() return 42; end;;
gap> # Use PrintToString() to define myfnc on all slave processes
gap> BroadcastMsg( PrintToString( "myfnc := ", myfnc ) );
gap> SendRecvMsg( "myfnc()", 1 );
42
gap> FlushAllMsgs(); # There are no messages pending.
0
gap> # Execute analogue of GAP's List() in parallel on slaves.
gap> squares := ParList( [1..100], x->x^2 );
[ 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256,
289, 324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841,
900, 961, 1024, 1089, 1156, 1225, 1296, 1369, 1444, 1521, 1600,
1681, 1764, 1849, 1936, 2025, 2116, 2209, 2304, 2401, 2500, 2601,
2704, 2809, 2916, 3025, 3136, 3249, 3364, 3481, 3600, 3721, 3844,
3969, 4096, 4225, 4356, 4489, 4624, 4761, 4900, 5041, 5184, 5329,
5476, 5625, 5776, 5929, 6084, 6241, 6400, 6561, 6724, 6889, 7056,
7225, 7396, 7569, 7744, 7921, 8100, 8281, 8464, 8649, 8836, 9025,
9216, 9409, 9604, 9801, 10000 ]
gap> # Ensure problem shared data is read into master and slaves.
gap> # Try one of your GAP program files instead.
gap> ParRead( "/home/gene/myprogram.g");
Now that you have done a fairly rudimentary test of ParGAP you should be ready to do something a little bit more interesting:
gap> ParInstallTOPCGlobalFunction( "MyParList", > function( list, fnc ) > local result, iter; > result := []; > iter := Iterator(list); > MasterSlave( function() if IsDoneIterator(iter) then return NOTASK; > else return NextIterator(iter); fi; end, > fnc, > function(input,output) result[input] := output; > return NO_ACTION; end, > Error > ); > return result; > end ); gap> MyParList( [1..25], x->x^3 ); master -> 1: 1 master -> 2: 2 2 -> master: 8 1 -> master: 1 master -> 1: 3 master -> 2: 4 2 -> master: 64 1 -> master: 27 master -> 1: 5 master -> 2: 6 2 -> master: 216 1 -> master: 125 master -> 1: 7 master -> 2: 8 2 -> master: 512 1 -> master: 343 master -> 1: 9 master -> 2: 10 2 -> master: 1000 1 -> master: 729 master -> 1: 11 master -> 2: 12 2 -> master: 1728 1 -> master: 1331 master -> 1: 13 master -> 2: 14 2 -> master: 2744 1 -> master: 2197 master -> 1: 15 master -> 2: 16 2 -> master: 4096 1 -> master: 3375 master -> 1: 17 master -> 2: 18 2 -> master: 5832 1 -> master: 4913 master -> 1: 19 master -> 2: 20 2 -> master: 8000 1 -> master: 6859 master -> 1: 21 master -> 2: 22 2 -> master: 10648 1 -> master: 9261 master -> 1: 23 master -> 2: 24 2 -> master: 13824 1 -> master: 12167 master -> 1: 25 1 -> master: 15625 [ 1, 8, 27, 64, 125, 216, 343, 512, 729, 1000, 1331, 1728, 2197, 2744, 3375, 4096, 4913, 5832, 6859, 8000, 9261, 10648, 12167, 13824, 15625 ] gap> ParInstallTOPCGlobalFunction( "MyParListWithAglom", > function( list, fnc, aglomCount ) > local result, iter; > result := []; > iter := Iterator(list); > MasterSlave( function() if IsDoneIterator(iter) then return NOTASK; > else return NextIterator(iter); fi; end, > fnc, > function(input,output) > local i; > for i in [1..Length(input)] do > result[input[i]] := output[i]; > od; > return NO_ACTION; > end, > Error, # Never called, can specify anything > aglomCount > ); > return result; > end ); gap> MyParListWithAglom( [1..25], x->x^3, 4 ); master -> 1: (AGGLOM_TASK): [ 1, 2, 3, 4 ] master -> 2: (AGGLOM_TASK): [ 5, 6, 7, 8 ] 1 -> master: [ 1, 8, 27, 64 ] 2 -> master: [ 125, 216, 343, 512 ] master -> 1: (AGGLOM_TASK): [ 9, 10, 11, 12 ] master -> 2: (AGGLOM_TASK): [ 13, 14, 15, 16 ] 1 -> master: [ 729, 1000, 1331, 1728 ] 2 -> master: [ 2197, 2744, 3375, 4096 ] master -> 1: (AGGLOM_TASK): [ 17, 18, 19, 20 ] master -> 2: (AGGLOM_TASK): [ 21, 22, 23, 24 ] 1 -> master: [ 4913, 5832, 6859, 8000 ] 2 -> master: [ 9261, 10648, 12167, 13824 ] master -> 1: (AGGLOM_TASK): [ 25 ] 1 -> master: [ 15625 ] [ 1, 8, 27, 64, 125, 216, 343, 512, 729, 1000, 1331, 1728, 2197, 2744, 3375, 4096, 4913, 5832, 6859, 8000, 9261, 10648, 12167, 13824, 15625 ]
If you wish an accelerated introduction to the models of parallel programming provided here, you might wish to read the beginning of Chapter Slave Listener through section Slave Listener Commands, and then proceed immediately to Chapter Basic Concepts for the TOP-C model (MasterSlave).
The ParGAP package was designed and written by Gene Cooperman, College of Computer Science, Northeastern University, Boston, MA, U.S.A.
If you use ParGAP to solve a problem then please send a short email to gene@ccs.neu.edu about it, and cite the ParGAP package as follows:
\bibitem[Coo99]{Coo99}
Cooperman, Gene,
{\sl Parallel GAP/MPI (ParGAP/MPI)}, Version 1,
College of Computer Science, Northeastern University, 1999,
\verb+http://www.ccs.neu.edu/home/gene/pargap.html+.
1.6 Invoking ParGAP with Remote Slaves
ParGAP, unlike GAP, must be invoked under a separate name. After
ParGAP has been installed, a script bin/pargap.sh will have been
created which (after any changes you needed to make; see
Section Installing ParGAP) you should use to invoke ParGAP. This is
similar to GAP_ROOT_DIR/bin/gap.sh that is used to invoke the
non-parallel GAP. Installers are encouraged to treat pargap.sh in
analogy to gap.sh. For example, if your site has copied gap.sh to
/usr/local/bin/gap, then you should also look for the pargap.sh
script as /usr/local/bin/pargap.
In addition, when pargap (we'll assume that's how ParGAP is invoked
at your site) is called, there must be a file, procgroup, in the
current directory, or alternatively, if you wish to use a single
procgroup file for all jobs, and that procgroup file is in /home/joe,
then you can alias pargap to pargap -p4pg /home/joe/procgroup.
The procgroup file has a simple syntax, taken from the MPICH
implementation of MPI (inherited from P4). A # in column 1 introduces
a comment line. The first non-comment line should be local 0, verbatim.
This line declares the master process as the local process. Other lines
are of the form:
host-machine 1 pargap-script
e.g.
regulus.ccs.neu.edu 1 /usr/local/bin/pargap
The first field is the hostname for a remote process. The second field
specifies one thread per process. (ParGAP recognizes only the value 1
for the second field.) The third field is an absolute pathname for
ParGAP, as it would be called on the remote process. Note that you can
repeat the same line twice if you want two remote ParGAP processes on
the same processor. The default procgroup provided in the distribution
will have lines of form:
localhost 1 path-of-provided-pargap.sh
If you change path-of-provided-pargap.sh to just, say, pargap, this
will work only if pargap is in your path on the remote machine shell
(localhost in this case), using your default shell. On most machines,
localhost is an alias for the local processor. This is a good default
for debugging, so that you don't disturb users on other machines.
MPI will use a line
host-machine 1 pargap-script
to create a UNIX subprocess executing:
rsh host-machine pargap-script
Suppose host-machine is regulus.ccs.neu.edu and pargap-script is
/usr/local/bin/pargap as in the above example, and we were to have
trouble invoking ParGAP, then it would be a good idea to try invoking
rsh regulus.ccs.neu.edu from a UNIX prompt and if that succeeds, to
then try executing the full rsh command.
A typical problem is that the remote processor requires a password to
login. MPI requires a login without passwords. Typically,
/etc/hosts.equiv has not been set up to remove the password requirement
for your remote host. Sometimes this can be solved by an appropriate
.rhosts file in your home directory on the remote host. Sometimes, PAM
is also used for user authentication (see /etc/pam.conf). man in.rshd
also has helpful information. Consult your system staff for further
analysis. In these days of hyper-security, rsh may be disabled at your
site and you may have to use ssh instead; if so, there is a solution
here: add the lines
############################################################################# ## ## RSH . . . .. . . . . . . . . . . . . . . . . remote shell used by ParGAP ## ## RSH=ssh export RSH
before the GAP block with the exec line. (Of course, the # lines
are not needed; they are comments.)
Note that the remote ParGAP process will not read from standard input,
although signals such as SIGINT (^C) may be received by the remote
process. However, the remote ParGAP process will write to standard
output, which is relayed to the local process. So,
gap> SendMsg("Exec(\"hostname\")", 2);
will execute and print from the remote process.
1.7 Problems with Installation
If you still have problems, here is a list of things to check.
lib/init.g file. Please add:
PAR_GAP_SLAVE_START := fail;
READ(GAP_RC_FILE);
if PAR_GAP_SLAVE_START <> fail then PAR_GAP_SLAVE_START(); fi;
top. The Linux
version of top sorts by memory usage if you type M.
make tries to automatically create:
pkg/pargap/bin/pargap.sh
/bin/gap.sh. GAP_ROOT was
specified when you executed ./configure GAP_ROOT/bin/gap.sh, does gap come up? If so, compare
it with pargap.sh and check for correct settings in
.../pkg/pargap/bin/pargap.sh?
procgroup file?
[It looks in the current directory for procgroup, or for:
... -p4pg PATH/procgroup
rsh remote-hostnamessh instead of rsh, then there is a
security issue. Read Section Problems with Passwords (Getting Around Security), and possibly man sshd.
man rshd tells you
the security model at your site (or possibly man ssh if you use
that). Then read Section Problems with Passwords (Getting Around Security).
procgroup file in your current directory set correctly?
Test it. If you are calling it on a remote host, manually type:
rsh HOSTNAME ParGAP
procgroup, e.g.
rsh denali.ccs.neu.edu /usr/local/gap4r3/bin/pargap.sh
exec is used to save process overhead. Also try:
rsh HOSTNAME exec ParGAP
/tmp/pargapmpi--rsh.$$
$$ is replaced by the the process id of the ParGAP
process.
pargap listed in .../pkg/ALLPKG?
[It's needed to autostart slaves.]
gap> MPI_Initialized();
cd to a directory of the same name as your local
directory. Check your assumptions about the remote machine. Try:
gap> SendRecvMsg("Exec(pwd)"); SendRecvMsg("UNIX_Hostname()");
gap> SendRecvMsg("UNIX_Getpid()");
SO_KEEPALIVE and variants.
(See man setsockopt.)
This periodically sends null messages so the remote machine does
not think that the originating machine is dead. However, if the
remote machine fails to reply, the local process sends a SIGPIPE
signal to notify current processes of a broken socket, even though
there might have been only a temporary lapse in connectivity.
ssh specifies KeepAlive yes by default, but setting KeepAlive no
might get you through some transient lapses in connectivity due to
high congestion.
You may also want to experiment with: setenv RSH "rsh -n"
1.8 Problems with Hosts on Multiple Networks
If a host is on multiple networks, it will have multiple IP addresses and
usually multiple hostnames. In this case, the master process cannot
always guess correctly which IP address (which internet address) should
be passed to the slave process, so that the slave process can call back
to the master. In such cases, you may need to tell ParGAP which
hostname or IP address to use for the callback. This is done by setting
the UNIX environment variable, CALLBACK_HOST, as in the example below.
# [ in sh/bash/... ] CALLBACK_HOST=denali.ccs.neu.edu; export CALLBACK_HOST # [ in csh/tcsh/... ] setenv CALLBACK_HOST=denali.ccs.neu.edu
The appropriate line for your shell can be placed in your shell
initialization file. Alternatively, you can set this up for all users by
placing the Bourne shell version (for sh) somewhere between the first
and last line of .../pkg/pargap/bin/pargap.sh.
1.9 Problems with Passwords (Getting Around Security)
There is a simple test to see if you need to read this section. Pick a
remote machine, HOSTNAME, that you wish to execute on, and type: rsh
HOSTNAME. If this did not work, also try ssh HOSTNAME. If you were
asked for your password, then you and your system administrator may need
to talk about security policy. If you were successful with ssh and not
with rsh then set the environment variable, RSH, to the value ssh,
as described in item 3 below.
hosts.equiv
file, so that logging in from one to the other does not require a
password. (man hosts.equiv)
.rhosts file to your home directory (or .shosts for ssh).
rsh to
start remote processes. However, if the environment variable RSH
was set, the script uses the value of the environment variable
instead of rsh. This may be useful, if you have your own script,
myrsh, that automatically gets around the security issues. Then
just type:
RSH=myrsh; export RSH # [ in sh/bash/... ] setenv RSH myrsh # [ in csh/tcsh/... ]
sh) somewhere between the
first and last line of .../pkg/pargap/bin/pargap.sh. (The example
for ssh was given earlier.)
ssh: man ssh mentions some possibilities for giving the password
the first time, and then having ssh remember that future logins to
that machine are authorized for the duration of the session. Don't
overlook the use of $HOME/.ssh/config to set special parameters,
such as specifying a different login name on the remote machine. Some
parameters of interest might be KeepAlive, RSAAuthentication,
UseRsh. You may also find useful information in man sshd.
/tmp/pargapmpi--rsh.$$
Note that this package modifies the GAP src and bin files, and
creates a new GAP kernel. This new GAP kernel can be shared by
traditional users of the old, sequential GAP kernel, and by those
doing parallel processing.
The GAP kernel will have identical behavior to the old GAP kernel
when invoked through the gap.sh script or the bin/@GAParch@/gap
binary. The new ParGAP variables will appear to the end user ONLY if
the GAP binary was invoked as pargapmpi: a symbolic link to the
actual GAP binary. The script, pargap.sh, does this.
So, in a multi-user environment, traditional users can continue to use
gap.sh without noticing any difference. Only an invocation of
pargap.sh will add the new features.
In a future version of GAP, it is hoped that the GAP kernel will
have enough ``hooks'', so that no modification of the GAP kernel is
required. At that time, it will also be possible to speed up the startup
time for ParGAP. Much of the startup time is caused by waiting for
GAP to read its library files. It will be possible to use the GAP
function, SaveWorkspace() to save a version with the GAP library
pre-loaded. That saved version can then be used to start up ParGAP.
This is not currently possible, because ParGAP needs to get at the
command line of GAP before the GAP kernel sees it.
Comments and contributions to a ParGAP user library, or any other type of assistance, are gratefully accepted.
Gene Cooperman gene@ccs.neu.edu
ParGAP manual