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)