This is the command pvm_shmdPVM that can be run in the OnWorks free hosting provider using one of our multiple free online workstations such as Ubuntu Online, Fedora Online, Windows online emulator or MAC OS online emulator
PROGRAM:
NAME
pvm_shmd - PVM shared memory (2) daemon
SYNOPSIS
pvm_shmd [ -options ] [ hostfile ]
DESCRIPTION
Pvm_shmd is a daemon process which maintains and handles the usage of shared memory
resources (memory segments, semaphores, message queues) on a particular PVM host. One
pvm_shmd must run on each host in a virtual machine that wants to use the shmd shared
memory message passing layer. The shmd message passing layer allows local processes to a
host to use shared memory for message passing on that host ONLY. See the shmd directory
README for further notes and restrictions.
The daemon can be started manually as with any other PVM task. Only one pvm_shmd can be
active per host. If the pvm_shmd detects another pvm_shmd it will shutdown.
The pvm_shmd can be killed from the PVM console by using the reset command. Local and
remote pvm_shmds can also be started from the PVM console program pvm.
The name of the daemon executable is pvm_shmd.
Shared Resource Usage
The pvm_shmd will attempt to create upto MAXSEG shared memory segments of upto MAXPAGES
memory pages in size. The size of each memory page is architecture dependent(see
getpagesize(3C)). The segments are then numbered 0..N-1 where N is the number of segments
created. The first segement (0) has the shared memory control structure placed in its
head. This structure allows alien processes to located the other shared memory segments
and any required controlling information.
The message passing layer, allocates memory from these segments asynchronously without any
interaction with the pvm_shmd using semaphores to protect data during updates to any
associated structures. For each segment there is an associated page map of which
processes have currently locked a page. Each page map for a segment has a separate
semaphore protecting it. The semaphores are accessed with the SEM_UNDO flag set so that if
a process holding a semaphore should die, the OS (should) reset the semaphore
automatically, thus allowing any waiting/blocked processes to continue.
The pvm_shmd only maintains the segments and their allocation page map(s). Thus if a
process allocated pages in a segment and then exits, it is the pvm_shmd that detects this
and then frees the allocated pages.
The pvm_shmd can have its status checked at any time by using the pvm_shmd_stat process to
kick it into reporting onto either stdio or the pvm log file (pvml.uid) its internal
state.
The shared memory and all associated processes can be cleared by sending the pvm_shmd a
HUP signal.
Sending the pvm_shmd a TERM signal will just cause it to clear any shared resources and
then exit.
The pvm_shmd cannot catch the KILL signal.
If the pvm_shmd is killed without clearing all of its shared resources these can be
cleared by calling ipcfree which resides in the pvm3/lib directory.
Message Passing using SHMD
The pvm_shmd handles resources that are used by special versions of pvm_psend() and
pvm_precv() stored in the libpvmshmd.a library. Thus to use these facilities, applications
have to link to this library instead of the usual libpvm3.a library.
OPTIONS
The following options may be specified on the command line when starting the pvm_shmd:
-debug=level
Sets the pvm_shmd debug level. Used to debug the pvm_shmd or libpvmshmd (not
intended to be used to debug application programs).
-maxsegs=maxsegs
Sets the maximum number of segments that the pvm_shmd can create. This is used to
over-ride the compiled in value from shmd.h. Note that the value cannot be above
the MAXSEGS in the shmd.h file.
-maxpages=maxpages
Sets the maximum segment size to maxpages pages of memory. This value cannot be
above the compiled value MAXPAGES in shmd.h or the actual OS defined limit.
NOTES
Remember that pvm_shmd allocated memory from the VM available on the machine. Allocating
more segements improves performance as there is less sharing of segments (semaphores for
their page maps). Although you must remember to leave some memory available for normal
program and OS system usage, as the pvm_shmd allocated memory is *ONLY* used for message
passing.
Use pvm_shmdPVM online using onworks.net services
