POSTED WRITE

1/25/97

POSTED WRITES: Posted writing is a technique used to improve the performance of caches. It works by deferring cache write operations triggered by memory reads that will use the same cache cell until after the read has been completed. Mid 1990 CPUs typically have 4 or 8 Posted write buffers. Posted writing is also used with write back caching and could be (not necessarily is) used to try to trickle "dirty" data back to memory during periods when the memory bus is unused. That would reduce the amount of "dirty" data that must be written in a burst when context is switched or the cache is flushed for IO.

Posted writing is subject to criticisms. Although the memory interface is unused when the posted write starts, that does not mean that it will not be needed for a read operation before the posted write is completed. In that case, the CPU may be stalled for a few cycles until the posted write is completed. Posted writing also makes overall operation more complex since the posted write buffer(s) add one more place which must be handled properly to maintain cache/memory coherency.

Actual improvements from Posted writing are difficult to estimate analytically and no figures are available on measured improvement. It is probably not large in most cases.

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