Re: [PATCH] ipc,shm: increase default size for shmmax

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 31 Mar 2014 15:59:33 -0700 Davidlohr Bueso <davidlohr@hp.com> wrote:

> > 
> > - Shouldn't there be a way to alter this namespace's shm_ctlmax?
> 
> Unfortunately this would also add the complexity I previously mentioned.

But if the current namespace's shm_ctlmax is too small, you're screwed.
Have to shut down the namespace all the way back to init_ns and start
again.

> > - What happens if we just nuke the limit altogether and fall back to
> >   the next check, which presumably is the rlimit bounds?
> 
> afaik we only have rlimit for msgqueues. But in any case, while I like
> that simplicity, it's too late. Too many workloads (specially DBs) rely
> heavily on shmmax. Removing it and relying on something else would thus
> cause a lot of things to break.

It would permit larger shm segments - how could that break things?  It
would make most or all of these issues go away?



First principles: why does this thing exist?  What problem was SHMMAX
created to solve?  It doesn't appear to be part of posix:

http://pubs.opengroup.org/onlinepubs/000095399/functions/shmget.html

[ENOMEM]
    A shared memory identifier and associated shared memory segment
    shall be created, but the amount of available physical memory is
    not sufficient to fill the request.