Intel ARCHITECTURE IA-32 User Manual download pdf (Page 309)

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101

Optimizing Cache Usage 6

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Hardware Prefetch

The automatic hardware prefetch, can bring cache lines into the unified

last-level cache based on prior data misses. The automatic hardware

prefetcher will attempt to prefetch two cache lines ahead of the prefetch

stream. This feature is introduced with the Pentium 4 processor.

The characteristics of the hardware prefetching are as follows:

• Requires some regularity in the data access patterns:

— if a data access pattern has constant stride, hardware prefetching

is effective only if access stride is less than half of the trigger

distance of hardware prefetcher (see Table 1-2).

— if access stride is not constant, the automatic hardware

prefetcher can mask memory latency if the strides of two

successive cache misses are less than the trigger threshold

distance (small-stride memory traffic).

— the automatic hardware prefetcher is most effective if the

strides of two successive cache misses remain less than the

trigger threshold distance and close to 64 bytes.

• Start-up penalty before hardware prefetcher triggers and extra

fetches after array finishes. For short arrays this overhead can

reduce effectiveness of the hardware prefetcher.

— The hardware prefetcher requires a couple misses before it

starts operating.

— Hardware prefetching will generate a request for data beyond

the end of an array, which will not be utilized. This behavior

wastes bus bandwidth. In addition this behavior results in a

start-up penalty when fetching the beginning of the next array;

this occurs because the wasted prefetch should have been used

instead to hide the latency for the initial data in the next array.

Software prefetching can recognize and handle these cases.

• Will not prefetch across a 4K page boundary; i.e., the program

would have to initiate demand loads for the new page before the

hardware prefetcher will start prefetching from the new page.

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Intel ARCHITECTURE IA-32 User Manual Page 309

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