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

[Kamezawa Hiroyuki]

(2014/04/02 4:19), Andrew Morton wrote:
> On Tue, 01 Apr 2014 15:29:05 +0900 Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
>
> > > So their system will act as if they had set SHMMAX=enormous.  What
> > > problems could that cause?
> > >
> > >
> > > Look.  The 32M thing is causing problems.  Arbitrarily increasing the
> > > arbitrary 32M to an arbitrary 128M won't fix anything - we still have
> > > the problem.  Think bigger, please: how can we make this problem go
> > > away for ever?
> >
> > Our middleware engineers has been complaining about this sysctl limit.
> > System administrator need to calculate required sysctl value by making sum
> > of all planned middlewares, and middleware provider needs to write "please
> > calculate systcl param by....." in their installation manuals.
>
> Why aren't people just setting the sysctl to a petabyte?  What problems
> would that lead to?

They(and admin) don't know the fact, setting petabytes won't cause any pain.

In their thinking:
==
If there is a kernel's limit, it should have some (bad) side-effect and
the trade-off which must be handled by admin is represented by the limit.
In this case, they think setting this value large will consume tons of resource.
==
They don't care kernel's implemenation but takes care of what API says.

Of course, always I was asked, I answer set it to peta-bytes. But the fact
*default is small* makes them doubtful.

Thanks,
-Kame

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