On Wed, Jul 15, 2026 at 03:48:38PM +0000, Taylor R Campbell wrote:
> > Date: Wed, 15 Jul 2026 11:42:00 -0400
> > From: Greg Troxel <[email protected]>
[..]
> > I propose to change SEMMNI from 10 to 32.
> > I propose to change SEMMNS from 60 to 256.
> >
> > The concept would be, as it is now, that individual ports or kernel
> > configs could just set these to lower values, if that's appropriate.
>
> Why not vary automatically with the amount of RAM at boot?
This could be done with a few #ifdef tricks. The numbers in struct seminfo
are all int32_t, so we could init them to -1 unless the option is defined
and overrides it, like:
/*
* Values in support of System V compatible semaphores.
*/
#ifdef SYSVSEM
struct seminfo seminfo = {
#ifdef SEMMAP
SEMMAP, /* # of entries in semaphore map */
#else
-1,
#endif
#ifdef SEMMNI
SEMMNI, /* # of semaphore identifiers */
#else
-1,
#endif
#ifdef SEMMNS
SEMMNS, /* # of semaphores in system */
#else
-1,
#endif
....
and then check for -1 in seminit() and set up a value based on available
RAM and KVA.
To get a concrete idea of what we are talking about, here are the
sizes on amd64:
/* Allocate the wired memory for our structures */
sz = ALIGN(seminfo.semmni * sizeof(struct semid_ds)) +
ALIGN(seminfo.semmns * sizeof(struct __sem)) +
ALIGN(seminfo.semmni * sizeof(kcondvar_t)) +
ALIGN(seminfo.semmnu * seminfo.semusz);
sz = round_page(sz);
and:
(gdb) p sizeof(struct semid_ds)
$2 = 64
(gdb) p sizeof(struct __sem)
$3 = 12
(gdb) p sizeof(kcondvar_t)
$4 = 16
so the suggested change costs 22*64 + 196*12 = 3760 bytes
plus maybe a bit of alignment - so 1 more wired page most likely.
However, most machines never need it (or not much of it), so I wonder if
we could (relatively simple) init it on first use only (and then maybe
scaled by KVA/RAM details).
Martin
