linux - How do I programmatically disable hardware prefetching?

Question

I would like to programmatically disable hardware prefetching.

From Optimizing Application Performance on Intel? Core? Microarchitecture Using Hardware-Implemented Prefetchers and How to Choose between Hardware and Software Prefetch on 32-Bit Intel? Architecture, I need to update the MSR to disable hardware prefetching.

Here is a relevant snippet:

"DPL Prefetch and L2 Streaming Prefetch settings can also be changed programmatically by writing a device driver utility for changing the bits in the IA32_MISC_ENABLE register – MSR 0x1A0. Such a utility offers the ability to enable or disable prefetch mechanisms without requiring any server downtime.

The table below shows the bits in the IA32_MISC_ENABLE MSR that have to be changed in order to control the DPL and L2 Streaming Prefetch:

Prefetcher Type MSR (0x1A0) Bit Value 
DPL (Hardware Prefetch) Bit 9 0 = Enable 1 = Disable 
L2 Streamer (Adjacent Cache Line Prefetch) Bit 19 0 = Enable 1 = Disable"

I tried using http://etallen.com/msr.html but this did not work. I also tried using wrmsr in asm/msr.h directly but that segfaults. I tried doing this in a kernel module ... and killed the machine.

BTW - I am using kernel 2.6.18-92.el5 and it has MSR linked in the kernel:

$ grep -i msr /boot/config-$(uname -r)
CONFIG_X86_MSR=y
...

Answer 1

You can enable or disable the hardware prefetchers using msr-tools http://www.kernel.org/pub/linux/utils/cpu/msr-tools/.

The following enables the hardware prefetcher (by unsetting bit 9):

[root@... msr-tools-1.2]# ./wrmsr -p 0 0x1a0 0x60628e2089 
[root@... msr-tools-1.2]# ./rdmsr 0x1a0 
60628e2089

The following disables the hardware prefetcher (by enabling bit 9):

[root@... msr-tools-1.2]# ./wrmsr -p 0 0x1a0 0x60628e2289 
[root@... msr-tools-1.2]# ./rdmsr 0x1a0 
60628e2289

Programatically, you can do this as root by opening /dev/cpu/<cpunumber>/msr and using pwrite to write to the msr "file" at the 0x1a0 offset.