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

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Date:	Mon, 31 Mar 2014 16:13:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Manfred Spraul <manfred@...orfullife.com>, aswin@...com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] ipc,shm: increase default size for shmmax

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

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