Re: shared memory settings on MAC OS X

Either way, when you do manage to change your shared memory settings,
you still won't be able to use more than about 4 GB of shared buffer
space in Postgres, or any other program for that matter. We thought
Leopard would help with this, but it hasn't. We have a 32 GB dual 3
Ghz Intel Xeon Xserve, but we can't set our shared buffers as high as
we'd like. We also have a similarly spec'd Linux machine and it has no
problem allowing us to specify lots of shared buffers. For some
reason, OS X gives an error when you try to attach to anything more
than 4 GB.

Here's a program we wrote to demonstrate the problem, which we have
submitted to Apple as a bug (Radar #5547628):

/*** create_shared_memory.c ***/
/* Takes as input from standard in a size in bytes for a shared memory
segment
that this program will attempt to create and attach to. If any errors
occur
trying to create or attach to the shared memory, they are printed to
standard
out. This program will then attempt to detach from and delete the
shared memory
it allocated and/or attached to as appropriate.

On Mac OS X 10.5 Server (9A557) on an Intel Quad core Xserve,
compiling this
program with gcc version i686-apple-darwin9-gcc-4.0.1 as follows
#gcc -arch x86_64 create_shared_memory.c
you will see the maximum shared memory segment that can be attached to
is
4292870144. I do not know if this shared memory limit applies to the
final
version of Leopard Server or on other hardware.

Note: You will need greater than 4GB of memory on your machine and
large enough
values set for kern.sysv.shmall and kern.sysv.shmmax to verify this
shared
memory limit.

Known limitations: This program does not provide any output if the
allocated
shared memory cannot be deallocated or if the attached shared memory
cannot be
detached.

Written by: Brian Wipf
ClickSpace Interactive Inc.
Date: October 18, 2007
*/

#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/errno.h>

typedef struct {
int iPID;
} shared_t;

main()
{
int key = 912367102; /* Some random key for the shared memory
segment. */
size_t size;
int shmflg=IPC_CREAT | 0600;
int shmid;
shared_t *shared_mem;

/* If you want to specify your own key, uncomment the next two lines
of code. */
/*
printf("Enter an integer key for shared memory segment: ");
scanf("%d", &key);
*/
printf("Enter size for shared memory segment (in bytes): ");
scanf("%ld", &size);

/* Create the shared memory segment */
printf("\nAttempting to create the shared memory segment...\n");
shmid=shmget(key, size, shmflg);

if (errno != 0) {
printf("There was an error trying to allocate the requested
amount of shared memory\n");
printf("errno: %d\n", errno);
} else {
printf("Shared memory created successfully\n");

/* Attach to the shared memory segment */
printf("\nAttempting to attach to the shared memory segment...
\n");
shared_mem = (shared_t *) shmat ( shmid, 0, 0);

if (errno != 0) {
printf("Error trying to attach to the shared memory segment.
\n");
printf("errno: %d\n\n", errno);
} else {
printf("Shared memory attached to successfully\n");

/* Detach from shared memory segment */
printf("\nDetaching from the shared memory segment\n\n");
shmdt(shared_mem);
}

/* Delete the shared memory segment */
printf("Deleting the shared memory segment\n");
shmctl(shmid, IPC_RMID, NULL);
}

return 0;
}

____________________________________________________________________
Brendan Duddridge | CTO | 403-277-5591 x24 | brendan(at)clickspace(dot)com

ClickSpace Interactive Inc.
Suite L100, 239 - 10th Ave. SE
Calgary, AB T2G 0V9

http://www.clickspace.com

On Oct 29, 2007, at 7:40 AM, Tom Lane wrote:

> Maximilian Tyrtania <mty(at)fischerappelt(dot)de> writes:
>> Tom, I suspect you have Mac OS X Server installed, right? That's
>> probably
>> why your /etc/sysctl.conf file mentions that /etc/sysctl-
>> macosxserver.conf
>> file, while mine doesn't.
>
> Uh, no, I'm looking at my laptop. Curious that yours has no reference
> to the other file.
>
>> And frankly, to me it looks as if it means "if there is a /etc/
>> sysctl.conf
>> file, then read it and accept its settings. Then overwrite the sysctl
>> settings with the default values, no matter what."
>
> You're forgetting the point I made that the first complete set of
> shmem
> settings wins. If we could change the settings on the fly after that,
> all this would be a whole lot easier, but the OSX kernel locks them
> down
> somehow.
>
> BTW, I dunno if you read awk at all, but that awk command effectively
> says "print lines that contain = and do not contain #". You didn't
> try
> appending comments to the setting lines in your file did you?
>
> regards, tom lane
>
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