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.

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)