L2 Streaming Prefetch from The Tech ARP BIOS Guide

L2 Streaming Prefetch - The Tech ARP BIOS Guide

L2 Streaming Prefetch

Common Options : Enabled, Disabled

 

Quick Review of L2 Streaming Prefetch

Intel Core processors have a hardware streaming prefetch mechanism that automatically fetches an extra 64-byte cache line whenever the processor requests for a 64-byte cache line. This reduces cache latency by making the next cache line immediately available if the processor requires it as well.

When enabled, the processor will retrieve the currently requested cache line, as well as the subsequent cache line.

When disabled, the processor will only retrieve the currently requested cache line.

In a desktop system, enabling this feature improves performance as there’s a high probability of the processor requiring the next cache line as well as the currently requested cache line. It is therefore recommended that you enable the L2 Streaming Prefetch BIOS feature in a desktop system.

But in a server, the probability of the next cache line being required by the processor is lower than that of a desktop system. The higher cache miss ratio inevitably leads to higher bus utilization, which reduces the processor’s performance.

You will need to evaluate the performance effect of L2 Streaming Prefetch on your server and determine if it should be disabled or enabled for better performance. But servers should generally disable this feature.

 

Details of L2 Streaming Prefetch

L2 Streaming Prefetch is a BIOS feature specific to processors based on the Intel Core microarchitecture.

These processors have a hardware streaming prefetch mechanism that automatically fetches an extra 64-byte cache line whenever the processor requests for a 64-byte cache line. To be more specific, the adjacent cache line prefetch mechanism automatically fetches the adjacent cache line, essentially delivering 128-bytes to the processor, even if the processor does not request for the subsequent cache line.

This reduces cache latency by making the next cache line immediately available if the processor requires it as well. However, this increases bus traffic and can actually reduce performance if the processor does not actually require the second cache line. So, it is a double-edge sword.

When enabled, the processor will retrieve the currently requested cache line, as well as the subsequent cache line.

When disabled, the processor will only retrieve the currently requested cache line.

In a desktop system, enabling this feature improves performance as there’s a high probability of the processor requiring the next cache line as well as the currently requested cache line. It is therefore recommended that you enable the L2 Streaming Prefetch BIOS feature in a desktop system.

But in a server, this feature may actually degrade performance since data requests in servers are of a more random nature. The probability of the next cache line being required by the processor is lower than that of a desktop system.

The higher cache miss ratio inevitably leads to higher bus utilization, no thanks to the fact that the processor prefetches an extra 64-byte cache line everytime it retrieves a single cache line. This increased bus utilitization reduces the processor’s performance.

You will need to evaluate the performance effect of L2 Streaming Prefetch on your server and determine if it should be disabled or enabled for better performance. But servers should generally disable this feature.

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