05-29-2007
Quote:
Originally Posted by Perderabo
I do not share your trepidation regarding the performance hit. This is virtually the definition of of an array reference is performed and I use arrays quite a bit. Switching your app entirely to arrays and never using pointers at all might actually improve performance provided that you use the optimizer. In any event, many implementations to not allow you to choose the address of a shared memory segment and portable code should not rely on having that option. Shared libraries are compiled using PIC (position independent code) despite the fact that there is often a minor performance hit with PIC. Shared data segments should also be position independent. It's the cost of doing business.
That works beautifully if I want to partition the shared memory up into several buckets and reference each bucket by its index. However, assuming the DB is made up of differently sized information, I must either pick a bucket size big enough to store anything (and waste space on smaller things) or I allocate dynamically sized buckets and pass around pointers (as indexes no longer work).
Essentially, I was thinking I could create a version of malloc that operated within a shared memory region and then use it to allocate items in the DB dynamically to be stored in a chained hash table.
The "performance hit" on pointers is that I need to store the "pointer" to the bucket that I allocated (via my malloc routine) in shared memory somehow. Either that pointer is a native pointer into shared memory, or it is an offset into shared memory that every time application code goes to access a pointer it'll need to perform a conversion routine against it to acquire its position independant address. This would be required for either the array or non-native pointer methods. I guess I could tell the application (in the non-native pointer method) that the shared memory is a huge array of characters and access pointers through an "index" into the character array cast to the appropriate data type...but this seems just as ugly.
Maybe creating an intermediate "malloc" library against a shared memory segment is silly...but I don't know of a better way to store various dynamically sized data into any memory segment without wasting space on static sized buckets.
Last edited by DreamWarrior; 05-29-2007 at 02:18 PM..
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shmop(2) System Calls Manual shmop(2)
Name
shmop, shmat, shmdt - shared memory operations
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
char *shmat (shmid, shmaddr, shmflg)
int shmid;
char *shmaddr;
int shmflg;
int shmdt (shmaddr)
char *shmaddr;
Description
The system call attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of
the calling process. The segment is attached at the address specified by one of the following criteria:
If shmaddr is equal to zero, the segment is attached at the first available address as selected by the system.
If shmaddr is not equal to zero and (shmflg & SHM_RND ) is true, the segment is attached at the address given by (shmaddr- (shmaddr modulus
SHMLBA )).
If shmaddr is not equal to zero and (shmflg & SHM_RND ) is false, the segment is attached at the address given by shmaddr.
The segment is attached for reading if (shmflg & SHM_RDONLY ) is true. Otherwise, it is attached for reading and writing.
The system call detaches from the calling process's data segment the shared memory segment located at the address specified by shmaddr.
Return Values
Upon successful completion, the return values are as follows:
o The system call returns the data segment start address of the attached shared memory segment.
o The system call returns a value of zero (0).
Otherwise, a value of -1 is returned, and errno is set to indicate the error.
Diagnostics
The system call fails and not attach the shared memory segment, if any of the following is true:
[EINVAL] The shmid is not a valid shared memory identifier.
[EACCES] Operation permission is denied to the calling process. For further information, see
[ENOMEM] The available data space is not large enough to accommodate the shared memory segment.
[EINVAL] The shmaddr is not equal to zero, and the value of (shmaddr- (shmaddr modulus SHMLBA )) is an illegal address.
[EINVAL] The shmaddr is not equal to zero, (shmflg & SHM_RND ) is false, and the value of shmaddr is an illegal address.
[EMFILE] The number of shared memory segments attached to the calling process would exceed the system imposed limit.
The fails and does not detach the shared memory segment if:
[EINVAL] The shmaddr is not the data segment start address of a shared memory segment.
See Also
execve(2), exit(2), fork(2), shmctl(2), shmget(2)
shmop(2)