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

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Date:	Tue, 1 Apr 2014 15:08:43 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Manfred Spraul <manfred@...orfullife.com>, aswin@...com,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] ipc,shm: increase default size for shmmax

On Tue, 01 Apr 2014 15:02:31 -0700 Davidlohr Bueso <davidlohr@...com> wrote:

> On Tue, 2014-04-01 at 14:48 -0700, Andrew Morton wrote:
> > On Tue, 1 Apr 2014 17:41:54 -0400 KOSAKI Motohiro <kosaki.motohiro@...il.com> wrote:
> > 
> > > >> > Hmmm so 0 won't really work because it could be weirdly used to disable
> > > >> > shm altogether... we cannot go to some negative value either since we're
> > > >> > dealing with unsigned, and cutting the range in half could also hurt
> > > >> > users that set the limit above that. So I was thinking of simply setting
> > > >> > SHMMAX to ULONG_MAX and be done with it. Users can then set it manually
> > > >> > if they want a smaller value.
> > > >> >
> > > >> > Makes sense?
> > > >>
> > > >> I don't think people use 0 for disabling. but ULONG_MAX make sense to me too.
> > > >
> > > > Distros could have set it to [U]LONG_MAX in initscripts ten years ago
> > > > - less phone calls, happier customers.  And they could do so today.
> > > >
> > > > But they haven't.   What are the risks of doing this?
> > > 
> > > I have no idea really. But at least I'm sure current default is much worse.
> > > 
> > > 1. Solaris changed the default to total-memory/4 since Solaris 10 for DB.
> > >  http://www.postgresql.org/docs/9.1/static/kernel-resources.html
> > > 
> > > 2. RHEL changed the default to very big size since RHEL5 (now it is
> > > 64GB). Even tough many box don't have 64GB memory at that time.
> > 
> > Ah-hah, that's interesting info.
> > 
> > Let's make the default 64GB?
> 
> But again, yet another arbitrary value...

Well, I'm assuming 64GB==infinity.  It *was* infinity in the RHEL5
timeframe, but infinity has since become larger so pickanumber.

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