Swap on video ram
Warning: unless your X11 driver can be made to use less ram than exists (can be detected) your X server will tend to crash when you try to use the same section of RAM to store textures (even Qt does it) as swap. Using a video driver that allows you to override videoram should increase stability (vga and radeon (xf86-video-ati) do at least). Even then, you or I might have made mistakes and you can still get crashes or freezes (that don't listen to ctrl-alt-backspace; use the magic sysrq).
Why Do it
The fast memory on your graphics card (if you have one) can be used as general ram (actually swap) by using the MTD subsystem of the kernel. If you have lots of videoram (256 MiB) this can increase system responsiveness (if you have around 256 MiB main ram too :).
Agp has slow reads; ~8 M/s. Making this less desirable.
MTD is now included in kernel26 as of 184.108.40.206-6.
When you are running a kernel with MTD modules, you have to load the modules specifying the pci address ranges that correspond to the ram on your video card.
check the ranges with
Then you look for the sections that name your video card as an example mine is here:
01:00.0 VGA compatible controller: ATI Technologies Inc RV350 AR [Radeon 9600] (prog-if 00 [VGA controller]) Subsystem: PC Partner Limited Unknown device 0830 Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx- Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx- Latency: 64 (2000ns min), Cache Line Size: 32 bytes Interrupt: pin A routed to IRQ 11 Region 0: Memory at e0000000 (32-bit, prefetchable) [size=256M] Region 1: I/O ports at c800 [size=256] Region 2: Memory at ff8f0000 (32-bit, non-prefetchable) [size=64K] Expansion ROM at ff8c0000 [disabled] [size=128K] Capabilities: <access denied> 01:00.1 Display controller: ATI Technologies Inc RV350 AR [Radeon 9600] (Secondary) Subsystem: PC Partner Limited Unknown device 0831 Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx- Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx- Latency: 64 (2000ns min), Cache Line Size: 32 bytes Region 0: Memory at d0000000 (32-bit, prefetchable) [size=256M] Region 1: Memory at ff8e0000 (32-bit, non-prefetchable) [size=64K] Capabilities: <access denied>
What's important is Region 0, (the biggest one)
Since the video card needs some ram to serve its normal purpose, so you need to do some calculations. The offsets are easy to calculate as powers of 2. The card should use the beginning of the address range as framebuffer, textures etc. when limited (otherwise you get X11 crashing).
Example: total 2^28 bytes (256M) videoram, leaving 2^24 (32M) for the normal function (less will work fine) The start range, is 2^24 bytes more than the start of the pci address range shown by lspci -vvv.
The end is your total minus the amount you left for the card.
Load the modules in /etc/rc.conf
MODULES=(otherModulesYouNeed slram mtdblock)
mkswap /dev/mtdblock0 && swapon /dev/mtdblock0 -p 10 #higher priority
Add this to /etc/modprobe.d/modprobe.conf
options slram map=VRAM,0xStartRange,+0xUsedAmount
Xorg Driver Config
To keep X stable, your video driver needs to be told to use less than the detected videoram. In the Device section of your /etc/X11/xorg.conf the declaration
Section "Device" Driver "radeon" # or whichever other driver you use VideoRam 32768 #other stuff EndSection
specifies that you use 32 MiB of ram. Other drivers might take the number for videoram as being in MiB (I think vga does), check manpages.
Checking the Swap space being used
The following command may help you getting the used swap in the different spaces like disk partitions, flash disks and possibly this example of the swap on video ram