PVM_SHMD(1PVM)							  PVM Version 3.4						    PVM_SHMD(1PVM)

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 cur-
       rently  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  wait-
       ing/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.

FILES

	 $PVM_ROOT/lib/$PVM_ARCH/pvm_shmd PVM shared memory daemon executable
	 $PVM_ROOT/shmd/shmd.h Shared Memory hard limits header file
	 /tmp/pvml.uid Pvmd runtime error log

SEE ALSO

       pvm(1PVM), pvmd(1PVM), getpagesize(3C), ipcs(1), msgctl(2), semctl(2), shmctl(2), signal(5)

G.E.Fagg							  11 March, 1999						    PVM_SHMD(1PVM)