disable prefetch

blupp75 · ‎11-22-2004

Hi,

I'm wondering if there is any way to disable the hardware prefetch on a Pentium M. I wish to do this to be able to evaluate the effectiveness of my software prefetching more easily.

Regards,

Per

Intel_Software_Netw1 · ‎01-25-2005

Greetings fromIntel Software NetworkSupport. We will check with our engineering contacts and let you know.

Regards,

Lexi S.

IntelSoftware NetworkSupport

http://www.intel.com/software

Contact us

Message Edited by intel.software.network.support on 11-30-2005 04:12 PM

Intel_Software_Netw1 · ‎02-07-2005

Here is the response we received from our engineering contacts:

The hardware prefetcher is already optimized and enabled in the Intel Pentium M Processor.

To answer the question, there is no way to disable/ override the hardware prefetcher using the IA32 instruction set via model specific registers (MSRs).

More information on the prefetcher is available at the following URLs:

How to Optimize Prefetch on 32-bit Intel Architecture - Optimize the use of prefetches in code for the Intel Pentium 4 processor and the Pentium M processor:
http://www.intel.com/cd/ids/developer/asmo-na/eng/microprocessors/ia32/pentium4/83886.htm

In most situations, the hardware prefetcher is likely to reduce system memory latency without explicit intervention from software prefetches. Using software prefetch instructions excessively or indiscriminately will inevitably cause performance penalties. This is because excessively or indiscriminately using software prefetch instructions wastes the command and data bandwidth of the system bus. Using software prefetches delays the hardware prefetcher from starting to fetch data needed by the processor core. It also consumes critical execution resources and can result in stalled execution. The guidelines for using software prefetch instructions are described in Chapter 2. The calculation of prefetch-ahead distance is discussed in Chapter 6.

User/Source Coding Rule 27. (M impact, L generality) Avoid excessive use of software prefetch instructions and allow automatic hardware prefetcher to work. Excessive use of software prefetches can significantly and unnecessarily increase bus utilization if used inappropriately.

IA-32 Software Architecture Optimization Reference Manual:
http://developer.intel.com/design/pentium4/manuals/index_new.htm

The IA-32 Intel Architecture Optimization Reference Manual provides information on Intel NetBurst microarchitecture and the Pentium M processor microarchitecture. It describes code optimization techniques to enable you to tune your application for highly optimized results when run on Pentium Intel Xeon and Pentium M processors. Download document number 248966 (File Name/Size: 24896611.pdf, 2.55 MB)