Unable to disable Hardware prefetcher in Core i7

Question

I am getting Error while trying to disable Hardware prefetcher in my Core i7 system. I am following the method as per the link How do I programmatically disable hardware prefetching?

In my system

grep -i msr /boot/config-$(uname -r)

CONFIG_X86_DEBUGCTLMSR=y

CONFIG_X86_MSR=y

CONFIG_SCSI_ARCMSR=m

Here is my error message

root@ ./rdmsr 0x1a0

850089

[root@ ./wrmsr -p 0 0x1a0 0x850289 (to disable hardware prefetcher in Core i7)

wrmsr:pwrite: Input/output error

I am getting same error for disabling Adjacent cache line prefetcher

Any idea how to resolve this problem ? Thanks in advance .

Brechtel

Out of curiosity: Why on earth would one want to disable the hardware prefetcher? – Piceous

To perform comparison study with and without hardware prefetching . – Brechtel

Prefetch may be painful for I/O intensive workloads by wasting memory bandwidth – Foulup

Answer 1

It turns out 0x1A0 bits 9 and 19 is the right MSR/bits for Pentium 4 and some other older models, but 0x1A4 bits 0-3 are the right MSR bits for the several recent models from Nehalem onwards. I tested 0x1A4 with a Skylake server and it worked as well, whereas 0x1A0 did not.

Quick documentation here: Disclosure of H/W prefetcher control on some Intel processors

There is more complete documentation about this and other MSRs on vol3 of the developer's manual: (click here). The document also confirms that 0x1A0 bits 9 and 19 were used for this in older processor models but 0x14A is used for more recent models.

The other MSR's look fairly interesting for experimenting too ('disable L3 cache' anyone?). Just make sure to look for the MSRs under the right processor family!

Armillia

Answer 2

MSR specifications are specific to Intel processor families as described in chapter 35 of the Intel Architectures Software Developer Manuals As consequence, you have to check first that you can disable through MSRs the prefetcher for your CPU and then check which bits of which register must be toggled for that.

For my processor, Intel Xeon 5650 (06_2CH family) the manual specifies the bits 10 to 8 of the register IA32_MISC_ENABLE at address 0x1A0 are reserved. I guess that this means I can't toggle prefetcher on and off through MSR. According to an answer from an Intel employee here: "Intel has not disclosed how to disable the prefetchers on processors from Nehalem onward. You'll need to disable the prefetchers using options in the BIOS."

On my workstation, running

sudo wrmsr -p 0 0x1a0 0x850289

results in:

wrmsr: CPU 0 cannot set MSR 0x000001a0 to 0x0000000000850289

but

sudo wrmsr -p 0 0x1a0 0x850088

works.

This seems to confirm that I can't disable prefetching using MSRs. Note that this error is defferent from yours, and after reading the Intel manual mentioned above chapter 35.9 it also seems that for your processor you can't disbale the prefetcher with MSRs.I dont know why we don't have the same error message, what is your version of the msr module and Linux kernel ? (modinfo msr)

As a consequence the BIOS maybe the only way to disable the different prefetchers, it's the case for my Xeon 5650.

Escobar

Thanks for your answer. Also Intel are not supporting BIOS option to disable prefetcher. – Brechtel

@Brechtel I am not sure to understand your comment, in my case I can disable preftching in the BIOS. What is the exact CPU model for your processor ? – Escobar

@ Manuel , My CPU model is Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz and there is no option available in BIOS to disable prefetching in my CPU model. – Brechtel

Have you checked that the msr module is loaded into your kernel ? sudo lsmod and if not sudo modprobe msr – Escobar

Before and After the command sudo modprobe msr, I executed sudo lsmod. I got the same output. Then I tried to disable hardware prefetcher again using above tools , but no success , same i/o error. – Brechtel

Have you also checked that /dev/cpu/XXXX/msr (XXXX = cpu number) files are present ? – Escobar

yes, it is there. I tried to view its contents using cat , it displaying some image characters . While I try to open using vi editor , it says not a file. – Brechtel

Ok. You can't open these files as it, they are binary file created by the kernel module msr, just to reflect the content of your cpu's MSR and to interact with the jernel module to ask for MSR changes. – Escobar

Maybe you can try to write other bits in the MSR – Escobar

I have tried other bits also, but i got same error for bit 9 (DPL (Hardware Prefetch) ) and 19 (L2 Streamer (Adjacent Cache Line Prefetch)). – Brechtel

see my updated answer: sudo wrmsr -p 0 0x1a0 0x850088 works for me. Can you try and report the output of modinfo msr ? – Escobar

./wrmsr -p 0 0x1a0 0x850088 works for me also, but while I run modinfo msr i got this error. modinfo: ERROR: Module msr not found. Although cat /dev/cpu/0/msr displaying output. – Brechtel

let us continue this discussion in chat – Escobar