Intel ARCHITECTURE IA-32 User Manual download pdf (Page 335)

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Optimizing Cache Usage 6

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The following examples of using prefetching instructions in the

operation of video encoder and decoder as well as in simple 8-byte

memory copy, illustrate performance gain from using the prefetching

instructions for efficient cache management.

Video Encoder

In a video encoder example, some of the data used during the encoding

process is kept in the processor’s second-level cache, to minimize the

number of reference streams that must be re-read from system memory.

To ensure that other writes do not disturb the data in the second-level

cache, streaming stores (

movntq) are used to write around all processor

caches.

The prefetching cache management implemented for the video encoder

reduces the memory traffic. The second-level cache pollution reduction

is ensured by preventing single-use video frame data from entering the

second-level cache. Using a non-temporal prefetch (

prefetchnta)

instruction brings data into only one way of the second-level cache, thus

reducing pollution of the second-level cache. If the data brought directly

to second-level cache is not re-used, then there is a performance gain

from the non-temporal prefetch over a temporal prefetch. The encoder

uses non-temporal prefetches to avoid pollution of the second-level

cache, increasing the number of second-level cache hits and decreasing

the number of polluting write-backs to memory. The performance gain

results from the more efficient use of the second-level cache, not only

from the prefetch itself.

Video Decoder

In the video decoder example, completed frame data is written to local

memory of the graphics card, which is mapped to

WC (Write-combining)

memory type. A copy of reference data is stored to the

WB memory at a

later time by the processor in order to generate future data. The

assumption is that the size of the reference data is too large to fit in the

processor’s caches. A streaming store is used to write the data around

the cache, to avoid displaying other temporal data held in the caches.

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