Re: shared memory settings: SHMMAX and SHMALL

Matthew Rice wrote:

> Tatsuo Ishii <t-ishii(at)sra(dot)co(dot)jp> writes:
> > > If I have 512MB of memory in my system, excluding swap
> > > space,
> > > what values do I want to set for SHMMAX and SHMALL?
> >
> > That depeneds on your kernel implemetaion and hardware. I found
> > followings in /usr/include/asm/shmparam.h on my Linux box.
> >
> > here PAGE_SIZE == 4096. So I guess we cannot have more than 128MB for
> > the shared memory on this platform if above comment is correct.
>
> You can recompile the kernel with the new defaults or just change them on
> a live system (kernel 2.2.x or higher) with something like:
>
> echo 419430400 >/proc/sys/kernel/shmall
> echo 419430400 >/proc/sys/kernel/shmmax
>
> You pick the numbers, of course.

Two questions jump to my mind.

1) You've allocated 419430400 bytes (400MB) to shared memory. If I am
planning on running, e.g., 10 postmasters, wouldn't I want to set SHMMAX to
40MB (400MB divided by 10), since SHMMAX is the max shared memory allotment
per running application?

2) I've read that SHMALL = (SHMMAX/PAGE_SIZE*SHMMNI). I haven't been able
to determine SHMMNI's value on my system. I am inferring that PAGE_SIZE = 1
and SHMMNI = 4096 based simply on other users' comments and my
/usr/include/asm/shmparam.h (reprinted below) which, frankly, I am not very
good at reading. Anyway, if this is the case, isn't it likely that
SHMALL is less than SHMMAX?

/*
* Format of a swap-entry for shared memory pages currently out in
* swap space (see also mm/swap.c).
*
* SWP_TYPE = SHM_SWP_TYPE
* SWP_OFFSET is used as follows:
*
* bits 0..6 : id of shared memory segment page belongs to (SHM_ID)
* bits 7..21: index of page within shared memory segment (SHM_IDX)
* (actually fewer bits get used since SHMMAX is so low)
*/

/*
* Keep _SHM_ID_BITS as low as possible since SHMMNI depends on it and
* there is a static array of size SHMMNI.
*/
#define _SHM_ID_BITS 9
#define SHM_ID_MASK ((1<<_SHM_ID_BITS)-1)

#define SHM_IDX_SHIFT (_SHM_ID_BITS)
#define _SHM_IDX_BITS 15
#define SHM_IDX_MASK ((1<<_SHM_IDX_BITS)-1)

/*
* _SHM_ID_BITS + _SHM_IDX_BITS must be <= 24 on the i386 and
* SHMMAX <= (PAGE_SIZE << _SHM_IDX_BITS).
*/

#define SHMMAX 0x2000000 /* max shared seg size (bytes) */
/* Try not to change the default shipped SHMMAX - people rely on it */

#define SHMMIN 1 /* really PAGE_SIZE */ /* min shared seg size (bytes) */
#define SHMMNI (1<<_SHM_ID_BITS) /* max num of segs system wide */
#define SHMALL /* max shm system wide (pages) */ \
(1<<(_SHM_IDX_BITS+_SHM_ID_BITS))
#define SHMLBA PAGE_SIZE /* attach addr a multiple of this */
#define SHMSEG SHMMNI /* max shared segs per process */

#endif /* _ASMI386_SHMPARAM_H */