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Date: Sat, 6 Jul 1996 00:22:25 -0700 (PDT)
From: Tom Bartol <bartol@salk.edu>
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To: hackers@freebsd.org
Subject: Shared Memory Questions
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Hello FreeBSD Hackers,

I have a few questions regarding shared memory in FreeBSD.  I've been
using FreeBSD 2.1.0-release since it came out in November, 1995 and think
is the best free unix for intels machines without a doubt.  Many thanks to
the FreeBSD team and hackers for your heroic contribution to the OS world!

Now for my questions:

I'm running an excellent image manipulation tool called "The Gimp" on a
P5/133 with 64MB RAM and FreeBSD 2.1.0-release.  This program makes
extensive use of shared memory for iterprocess communication with external
image manipulation modules called "plug-ins".  These plug-ins are user
customizable and give the gimp incredible power and potential.  The
documentation with the gimp talks about how most unix varieties have
default shared memory parameters which are set too low to handle many of
the things that the gimp needs to do, especially when you try to operate
on large images.  Sure enough, on my machine I run out of shared memory
when I try to manipulate or load large images. 

ipcs -M gives me the following output:

shminfo:
        shmmax: 4194304 (max shared memory segment size)
        shmmin:       1 (min shared memory segment size)
        shmmni:      32 (max number of shared memory identifiers)
        shmseg:       8 (max shared memory segments per process)
        shmall:    1024 (max amount of shared memory in pages)

which I suppose is telling me that my max seg size is only ~4MB, and since
the pagesize command tells me that a page is only 4096 bytes I can only
allocate a total of 1024*4096 bytes in shared memory anyway!  I've poked
around in the kernel sources and FAQ's that come with 2.1.0-release and
have found only one small FAQ regarding shared memory, namely that in 
addition to enabling shared memory with the SYSVSHM kernel config
option there is an option called SHMMAXPGS which can be set thusly:

options         "SHMMAXPGS=64"

The FAQ shows shared memory being set to 64 pages, or 256KB -- not quite
big enough.  The LINT kernel config file does not list this option.  Upon
further investigation I found a kernel source file called
/usr/src/sys/conf/param.c which sets the default values for shmmax,
shmmin, shmmni, shmseg, and shmall reported by the ipcs -M command as
above.  Setting SHMMAXPGS to 16384 gave me the expected result of
increasing available shared memory to 64MB and I can now open large images
but gimp now sometimes runs short on shmseg.  I tried increasing this
parameter by editing param.c directly and setting shmseg to a nice round
figure of 128.  The kernel this generated was unstable in that the ipcs -b
command would sometimes seg fault and would report nonsense values for
which shared memory segments were currently in use etc...  I reduce shmseg
to 16 which SEEMS to be working O.K. for now, though I still run out of
segs in gimp from time to time, but I'd like to be able to choose and tune
the shared memory parameters at will, and to do so with config options if
possible.  Or better yet, why can't the kernel just allow these
resources to grow without bound until RAM and VM are exhausted?

So, the bottom line is this -- can any of you give some advice on how to
tune shared memory in FreeBSD?  Are these numbers meant to be hard coded
and un-tweekable?  Can it be done at all?  I really know next to nothing
about shared memory and any help you can give would be greatly
appreciated (virtual beers etc...)  :) 

Thanx,

Tom