Intel IXP2800 , 3.8.1.5 Stall/Writeback Statistics,

110 Hardware Reference Manual
Intel® IXP2800 Network Processor
Intel XScale® Core
Statistics derived from these two events:
•The average number of cycles the processor stalled on a data-cache access that may overflow
the data-cache buffers.
This is calculated by dividing PMN0 by PMN1. This statistic lets you know if the duration
event cycles are due to many requests or are attributed to just a few requests. If the average is
high, the Intel XScale® core may be starved of the bus external to the Intel XScale® core.
•The percentage of total execution cycles the processor stalled because a Data Cache request
buffer was not available.
This is calculated by dividing PMN0 by CCNT, which was used to measure total execution
time.
3.8.1.5 Stall/Writeback Statistics
When an instruction requires the result of a previous instruction and that result is not yet available,
the Intel XScale® core stalls, to preserve the correct data dependencies. PMN0 counts the number
of stall cycles due to data dependencies. Not all data dependencies cause a stall; only the following
dependencies cause such a stall penalty:
•Load-use penalty: attempting to use the result of a load before the load completes. To avoid the
penalty, software should delay using the result of a load until it’s available. This penalty shows
the latency effect of data-cache access.
•Multiply/Accumulate-use penalty: attempting to use the result of a multiply or multiply-
accumulate operation before the operation completes. Again, to avoid the penalty, software
should delay using the result until it’s available.
•ALU use penalty: there are a few isolated cases where back-to-back ALU operations may
result in one cycle delay in the execution.
PMN1 counts the number of writeback operations emitted by the data cache. These writebacks
occur when the data cache evicts a dirty line of data to make room for a newly requested line or as
the result of clean operation (CP15, register 7).
Statistics derived from these two events:
•The percentage of total execution cycles the processor stalled because of a data dependency.
This is calculated by dividing PMN0 by CCNT, which was used to measure total execution
time. Often, a compiler can reschedule code to avoid these penalties when given the right
optimization switches.
•Total number of data writeback requests to external memory can be derived solely with PMN1.