09-21-2001
If you and I are on the system and we both run vi to edit different files, no modern kernel will give us two complete seperate copies of vi. The program's memory will be broken into segments. The text segment contains (roughly) the code and the unchanging constants. We will share one text segment bewteen us. We each get a data segment and we each get a stack. top will report the total of all the segments for each process, but you can't add them up and expect that system needs to use that much physical memory. With shared libraries, it get a bit more confused. If you use emacs and I use vi, will we have different text segments, but we both might be using the same copy of printf and other library functions. Oracle is probably using shared memory which further complicates things. Shared memory is another memory segment, and again many processes can map that seqment into their memory space. So 100 copies of a 700 MB program might easily fit in a couple of GB of memory.
In view of all this, it really isn't possible anymore to truly say how much memory a single process is using in the sense that you want. Each process really is using 700MB, but they are sharing very large chucks of this with other processes.
Most versions of top have a line that shows system-wide memory usage.
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LEARN ABOUT OPENDARWIN
shmdt
SHMAT(2) BSD System Calls Manual SHMAT(2)
NAME
shmat, shmdt -- map/unmap shared memory
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
void *
shmat(int shmid, void *shmaddr, int shmflg);
int
shmdt(void *shmaddr);
DESCRIPTION
shmat() maps the shared memory segment associated with the shared memory identifier shmid into the address space of the calling process. The
address at which the segment is mapped is determined by the shmaddr parameter. If it is equal to 0, the system will pick an address itself.
Otherwise, an attempt is made to map the shared memory segment at the address shmaddr specifies. If SHM_RND is set in shmflg, the system will
round the address down to a multiple of SHMLBA bytes (SHMLBA is defined in <sys/shm.h> ). A shared memory segment can be mapped read-only by
specifying the SHM_RDONLY flag in shmflg. shmdt() unmaps the shared memory segment that is currently mapped at shmaddr from the calling
process' address space. shmaddr must be a value returned by a prior shmat() call. A shared memory segment will remain existant until it is
removed by a call to shmctl(2) with the IPC_RMID command.
RETURN VALUES
shmat() returns the address at which the shared memory segment has been mapped into the calling process' address space when successful,
shmdt() returns 0 on successful completion. Otherwise, a value of -1 is returned, and the global variable errno is set to indicate the error.
ERRORS
shmat() will fail if:
[EACCES] The calling process has no permission to access this shared memory segment.
[ENOMEM] There is not enough available data space for the calling process to map the shared memory segment.
[EINVAL] shmid is not a valid shared memory identifier. shmaddr specifies an illegal address.
[EMFILE] The number of shared memory segments has reached the system-wide limit.
shmdt() will fail if:
[EINVAL] shmaddr is not the start address of a mapped shared memory segment.
SEE ALSO
shmctl(2), shmget(2), mmap(2)
BSD
August 17, 1995 BSD