FAQ? How do I use slram?
grmd at lotwillow.co.uk
Fri Jan 18 17:43:30 EST 2002
On Wednesday 26 December 2001 16:54 pm, I wrote:
>I understand that slram can be used to support using uncached RAM for
>swapping but I do not know how to set it up.
With many thanks to Jochen Schaeuble, I have this working correctly.
1. compile slram.c, mtdcore.c and mtdblock.c
I compiled these as modules rather than as part of a new kernel
and placed the object files with the other module objects that
came as part of the distribution I use.
slram.o and mtdblock.o both depend on mtdcore.o, slram appears
to provide access to the designated memory, while mtdblock is
used to provide a block interface for mtd devices.
I did 'depmod -a' so I could check that the dependancies were
recorded correctly in /lib/modules/.../modules.dep
2. choose options for slram.o
My computers are pentium-based using FX, VX and TX chipsets.
These chipsets only cache the first 64 Mbytes of RAM, and I had
enough memory to fit 128 Mbytes in some of them. I decided that
the second 64 MBytes was to be used by the slram module, so I
added the following to /etc/modules.conf
options slram map=mtdblock0,64M,+64M
which says that the device is to be /dev/mtdblock0, the block
of memory is to start at 64 Mbytes, and the size is 64 Mbytes.
Because there is no direct dependancy link between slram.o and
mtdblock.o I added
above slram mtdblock
to /etc/modules.conf so that mtdblock is loaded whenever the
other two modules are loaded.
3. add a new device
As mentioned previously, mtdblock.o provides a block interface
and I configured slram.o to use /dev/mtdblock0, so the device
must be created. The major number is 31, and the minor numbers
are assigned in pairs, even for read-write devices and odd for
read-only devices. As I required read-write access, I created
the block device
mknod /dev/mtdblock0 b 31 0
When the modules are loaded correctly, this device can be used
as any other block device, formatted, mounted, and so on.
4. add an extra boot option
I use the grub loader, and this requires that if the kernel is
to use less than the total available memory, grub must also be
told. I copied the default configuration and modified the copy
by giving it a different name, inserting the option to tell
grub that upper memory (memory above 1 Mbyte) was reduced, and
modified the kernel options
kernel /boot/vmlinuz-2.4.9-13 ro root=/dev/sda3 \
mem=exactmatch mem=640k at 0 mem=63M at 1M
Note that the kernel options were on one line, which I split
here for tidyness.
5. reboot and test the new configuration
I thought I'd test the configuration rather than set it up so
that it fired up automatically. Once I'd checked that the
kernel was using the correct amount of memory, (using the free
command), I did
I checked that all 3 modules were loaded by looking at
/proc/modules. If there were errors when loading, I could look
at the end of the output from dmesg to find the reason, but I
would probably need Jochen to explain what the messages mean!
Then I was able to use the /dev/mtdblock0 device. I did
which formatted the 64 Mbytes as a swap partition, then
swapon -p 10 /dev/mtdblock0
which enabled swapping to this device at a higher priority
than the disk partitions. When I was satisfied that this
worked properly, I added the commands to /etc/rc.d/rc.sysinit
so that the swap space would be created at power-on.
Problems I had:
1. The slram module emits an error if the memory that it has been
told to use is not available. This completely stopped me as I
didn't realise this was the problem.
2. When using grub to boot linux, any memory restriction that is
passed to the kernel must also passed to grub (in a suitable
manner), otherwise (i) grub loads the initialisation ramdisk
at the wrong location, and (ii) seems to inhibit slram from
loading. The grub 'uppermem' option takes a parameter in
kbytes above 1 Mbyte. It uses this to decide where to locate
the initialisation ramdisk. Get this wrong, and the kernel
will fail to boot!
3. To restrict the kernel to 64 Mbytes, I expected to be able to
add mem=64M to the kernel command line. When I did this, the
kernel used 384 kbytes more than expected. It might something
strange about the first motherboard I used, though. After some
research I found that I was able to define memory regions, see
the line in step 4, above.
4. The mtdblock module is required to access the device referred
to in the slram module options.
Hope this helps others,
Gareth Davies, Lotwillow Ltd. 'Always' and 'never' are two words you
Tel. +44 1344 294900 should always remember never to use.
Fax. +44 1344 294902 - M. Kendig
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