[Devel] Re: [Bugme-new] [Bug 11381] New: default shmmax

[adobriyan at gmail.com]

On Wed, Aug 20, 2008 at 12:00:43PM -0700, Andrew Morton wrote:
> 
> (switched to email.  Please respond via emailed reply-to-all, not via the
> bugzilla web interface).
> 
> On Wed, 20 Aug 2008 05:58:57 -0700 (PDT)
> bugme-daemon at bugzilla.kernel.org wrote:
> 
> > http://bugzilla.kernel.org/show_bug.cgi?id=11381
> > 
> >            Summary: default shmmax
> >            Product: Other
> >            Version: 2.5
> >      KernelVersion: 2.6.26.2
> >           Platform: All
> >         OS/Version: Linux
> >               Tree: Mainline
> >             Status: NEW
> >           Severity: enhancement
> >           Priority: P1
> >          Component: Other
> >         AssignedTo: other_other at kernel-bugs.osdl.org
> >         ReportedBy: peter_e at gmx.net
> > 
> > 
> > I would like to request that the default shmmax setting be increased or the
> > downsides of that be documented.  Allow me to explain.
> > 
> > I am with the PostgreSQL development team.  PostgreSQL is probably one of the
> > few users of large amounts of SysV shared memory.  Users would usually want to
> > configure anywhere between 10% and 50% of their physical RAM to be used as
> > shared memory, which would translate to something on the order of gigabytes
> > nowadays.  One of the uniformly annoying things about setting this up is that
> > you need to reconfigure the Linux kernel to allow that.  sysctl is nice and
> > all, but it still requires users to learn about operating system and kernel
> > details, requires root access, and distros don't handle sysctl uniformly
> > either.  Maybe there is even a good reason for that, but I couldn't find it,
> > and at least I would like to learn it, so that we can pass that information on
> > to our users.
> > 
> > I did some kernel version archeology and found out that up until kernels 2.2
> > the shmmax setting appears to have been restricted by CPU-specific constraints,
> > as indicated by the default setting being different across CPUs and being
> > defined in an asm header.  The default setting on i386 was increased from 16 MB
> > to 32 MB somewhere around 1998 in the kernel 2.0 line, and it remains at 32 MB
> > in the latest kernel on all architectures.
> > 
> > Now one question is whether there is a space or time overhead involved with
> > setting a high shmmax limit that isn't actually used.  If so, it would be
> > interesting to know what that overhead is.  The feeling I get from browsing the
> > kernel source code over time is that there was some management overhead and/or
> > some restrictions about this in old kernels, but that nowadays it doesn't
> > really seem to matter much anymore.  I suspect instead that this whole thing
> > was just forgotten, because few applications use large amounts of shared
> > memory.
> > 
> > So, if you want to do us a favor, could you please see about increasing the
> > default shmmax setting to whatever the theoretical maximum is?
> > 
> 
> I don't think anybody has even thought about the shmmax default in
> years.  Sure, it might be time to reexamine that.
> 
> It would be useful to get distro input on this.  Do they override the
> kernel default at boot time?  If so, what do they do?
> 
> 
> Also, from a quick read it looks to me that shmmax is busted in the
> non-init namespace.
> 
> clone_ipc_ns() calls shm_init_ns() which does
> 
> 	ns->shm_ctlmax = SHMMAX;
> 
> which a) fails to inherit the parent's setting and

This is debatable if such behaviour should be default, this makes one ipc_ns
more ipc_ns than others.

> b) cannot be altered from SHMMAX via the sysctl?

It can be altered. See kludge called get_ipc().

(well, I haven't checked personally, but it's a bug if IPC sysctls
aren't independently controllable after CLONE_NEWIPC, complain loudly).

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