Doc / mtrr

Doc / mtrr
2010-03-15 (月) 10:04:30更新

8.1. Setting up MTRR

It is VERY recommended to check if the MTRR registers are set up properly, because they can give a big performance boost.

Do a cat /proc/mtrr:

--($:~)-- cat /proc/mtrr
reg00: base=0xe4000000 (3648MB), size=  16MB: write-combining, count=9
reg01: base=0xd8000000 (3456MB), size= 128MB: write-combining, count=1

It's right, shows my Matrox G400 with 16MB memory. I did this from XFree 4.x.x , which sets up MTRR registers automatically.

If nothing worked, you have to do it manually. First, you have to find the base address. You have 3 ways to find it:

  1. from X11 startup messages, for example:
    (--) SVGA: PCI: Matrox MGA G400 AGP rev 4, Memory @ 0xd8000000, 0xd4000000
    (--) SVGA: Linear framebuffer at 0xD8000000
  2. from /proc/pci (use lspci -v command):
    01:00.0 VGA compatible controller: Matrox Graphics, Inc.: Unknown device 0525
    Memory at d8000000 (32-bit, prefetchable)
  3. from mga_vid kernel driver messages (use dmesg):
    mga_mem_base = d8000000

Then let's find the memory size. This is very easy, just convert video RAM size to hexadecimal, or use this table:

1 MB0x100000
2 MB0x200000
4 MB0x400000
8 MB0x800000
16 MB0x1000000
32 MB0x2000000

You know base address and memory size, let's setup MTRR registers! For example, for the Matrox card above (base=0xd8000000) with 32MB ram (size=0x2000000) just execute:

echo "base=0xd8000000 size=0x2000000 type=write-combining" >| /proc/mtrr

Not all CPUs have MTRRs. For example older K6-2 (around 266MHz, stepping 0) CPUs don't have MTRRs, but stepping 12 does (execute cat /proc/cpuinfo to check it).