Path: utzoo!attcan!uunet!virtech!cpcahil
From: cpcahil@virtech.uucp (Conor P. Cahill)
Newsgroups: comp.unix.wizards
Subject: Re: shared memory
Keywords: portability
Message-ID: <1989Dec12.053453.497@virtech.uucp>
Date: 12 Dec 89 05:34:53 GMT
References: <11383@csli.Stanford.EDU> <1989Dec12.005555.20618@virtech.uucp> <21223@mimsy.umd.edu>
Organization: Virtual Technologies Inc.
Lines: 35

In article <21223@mimsy.umd.edu>, chris@mimsy.umd.edu (Chris Torek) writes:
> In article <1989Dec12.005555.20618@virtech.uucp> cpcahil@virtech.uucp
> (Conor P. Cahill) writes:
> >Modifying the SHMMAX should only require a kernel re-configuration which
> >should always be an option.
> 
> As I understand it---which is not to say that it is so, for I have
> never seen the SysRel% 1, 2, or 3# code itself---the total amount of
> shared memory allowed per-system is reserved at boot time, is not
> pageable, and is effectively taken away from the rest of the system.
> For processes not using it, it is as if some of the machine's memory
> had been physically removed.
> 
> This would mitigate against raising SHMMAX arbitrarily....

SHMMAX is the maximum size of a single segment, not the total amount
of shared memory system wide, so raising it should not matter.

I don't believe that shared memory is reserverd at boot time because
I worked with a project that implemented shared libraries using shared 
memory segments and we maxed out all shared memory configuration 
options without it having a detrimental effect on memory available in
the system.  Another issue is that the pagability of shared memory is
controlled by a shmctl() call to lock/unlock a segment in memory.

In a quick test, I am able to create 15 meg of shared memory segments
on a system that has only 12 meg of memory, so there cannot be a boot
time reservation of the memory for shared memory. (At least this is
so under System V/386 Rel 3.2.  I believe this is true for most
implementations).
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| Conor P. Cahill     uunet!virtech!cpcahil      	703-430-9247	!
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