Memory Performance Considerations; Caching And Memory Coherency - IBM PowerPC 750GX User Manual

6.5 Memory Performance Considerations

Because the 750GX can have a maximum instruction throughput of three instructions per clock cycle, lack of
memory bandwidth can affect performance. For the 750GX to maximize performance, it must be able to read
and write data efficiently. If a system has multiple bus devices, one of them might experience long memory
latencies while another bus master (for example, a direct memory-access controller) is using the external
bus.

6.5.1 Caching and Memory Coherency

To minimize the effect of bus contention, the PowerPC Architecture defines WIM bits that are used to
configure memory regions as caching enforced or caching inhibited. Accesses to such memory locations
never update the L1 cache. If a cache-inhibited access hits the L1 cache, the cache block is invalidated. If the
cache block is marked modified, it is copied back to memory before being invalidated. Where caching is
permitted, memory is configured as either write-back or write-through, which are described as follows:
Write-back
Write-through
Chapter 3, Instruction-Cache and Data-Cache Operation, on page 121 describes the caches, memory config-
uration, and snooping in detail.
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March 27, 2006
Configuring a memory region as write-back lets a processor modify data in the
cache without updating system memory. For such locations, memory updates
occur only on modified cache-block replacements, cache flushes, or when one
processor needs data that is modified in another's cache. Therefore, configuring
memory as write-back can help when bus traffic could cause bottlenecks, espe-
cially for multiprocessor systems and for regions in which data, such as local vari-
ables, is used often and is coupled closely to a processor.
If multiple devices use data in a memory region marked write-through, snooping
must be enabled to allow the copy-back and cache invalidation operations neces-
sary to ensure cache coherency. The 750GX's snooping hardware keeps other
devices from accessing invalid data. For example, when snooping is enabled, the
750GX monitors transactions of other bus devices. If another device needs data
that is modified on the 750GX's cache, the access is delayed so the 750GX can
copy the modified data to memory.
Store operations to memory marked write-through always update both system
memory and the L1 cache on cache hits. Because valid cache contents always
match system memory marked write-through, cache hits from other devices do not
cause modified data to be copied back as they do for locations marked write-back.
However, all write operations are passed to the bus, which can limit performance.
Load operations that miss the L1 cache must wait for the external store operation.
Write-through configuration is useful when cached data must agree with external
memory (for example, video memory), when shared (global) data might be needed
often, or when it is undesirable to allocate a cache block on a cache miss.
IBM PowerPC 750GX and 750GL RISC Microprocessor
User's Manual
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