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- Intel Auto Counter Reload Support
- ---------------------------------
- Support for Intel Auto Counter Reload in perf tools
- Auto counter reload provides a means for software to specify to hardware
- that certain counters, if supported, should be automatically reloaded
- upon overflow of chosen counters. By taking a sample only if the rate of
- one event exceeds some threshold relative to the rate of another event,
- this feature enables software to sample based on the relative rate of
- two or more events. To enable this, the user must provide a sample period
- term and a bitmask ("acr_mask") for each relevant event specifying the
- counters in an event group to reload if the event's specified sample
- period is exceeded.
- For example, if the user desires to measure a scenario when IPC > 2,
- the event group might look like the one below:
- perf record -e {cpu_atom/instructions,period=200000,acr_mask=0x2/, \
- cpu_atom/cycles,period=100000,acr_mask=0x3/} -- true
- In this case, if the "instructions" counter exceeds the sample period of
- 200000, the second counter, "cycles", will be reset and a sample will be
- taken. If "cycles" is exceeded first, both counters in the group will be
- reset. In this way, samples will only be taken for cases where IPC > 2.
- The acr_mask term is a hexadecimal value representing a bitmask of the
- events in the group to be reset when the period is exceeded. In the
- example above, "instructions" is assigned an acr_mask of 0x2, meaning
- only the second event in the group is reloaded and a sample is taken
- for the first event. "cycles" is assigned an acr_mask of 0x3, meaning
- that both event counters will be reset if the sample period is exceeded
- first.
- ratio-to-prev Event Term
- ------------------------
- To simplify this, an event term "ratio-to-prev" is provided which is used
- alongside the sample period term n or the -c/--count option. This would
- allow users to specify the desired relative rate between events as a
- ratio. Note: Both events compared must belong to the same PMU.
- The command above would then become
- perf record -e {cpu_atom/instructions/, \
- cpu_atom/cycles,period=100000,ratio-to-prev=0.5/} -- true
- ratio-to-prev is the ratio of the event using the term relative
- to the previous event in the group, which will always be 1,
- for a 1:0.5 or 2:1 ratio.
- To sample for IPC < 2 for example, the events need to be reordered:
- perf record -e {cpu_atom/cycles/, \
- cpu_atom/instructions,period=200000,ratio-to-prev=2.0/} -- true
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