realtime:documentation:howto:tools:perf-smi-cost
Differences
This shows you the differences between two versions of the page.
| Both sides previous revision Previous revision | Next revision Both sides next revision | ||
|
realtime:documentation:howto:tools:perf-smi-cost [2018/07/13 11:02] ebugden Remove s in SMIs from link |
realtime:documentation:howto:tools:perf-smi-cost [2018/07/16 11:54] ebugden Remove inaccurate setup section |
||
|---|---|---|---|
| Line 10: | Line 10: | ||
| Add instructions (or link to instructions) for building perf from source? | Add instructions (or link to instructions) for building perf from source? | ||
| </WRAP> | </WRAP> | ||
| - | |||
| - | |||
| - | ===== Setup ===== | ||
| - | |||
| - | Because of how the average SMI duration is calculated, it is important to make sure that the CPUs never go idle while Perf is taking measurements, otherwise the results will be invalid. Before starting the measurements, the tool configures the core counters in the CPUs to stop counting cycles when an SMI is being handled. At the end of the test, the number of cycles spent handling SMIs is determined by calculating the difference between the /msr/aperf/ register (which never stops counting cycles) and the number of unhalted core cycles provided by the core counters (which stop counting during SMIs). However, a CPU's core counter also stop counting if the CPU goes idle. So, if CPUs are allowed to go idle, the calculated number of cycles spent handling SMIs will be larger than what it was in reality because it will include the time during which the CPUs were idle. | ||
| - | |||
| - | That being said, even if the CPUs never go idle and never have to handle an SMI (which means, in theory, that the total cycles and the unhalted cycles should be the same), it is alright if the value of /msr/aperf/ is a few thousand cycles higher than the count of unhalted core cycles. This is because the values of these counters cannot be updated atomically, so it is normal if the values are a little different. | ||
| - | |||
| - | Some simple ways of keeping the CPUs busy are described [[realtime:documentation:howto:debugging:no-cpu-idle|here]]. | ||
| ===== Test ===== | ===== Test ===== | ||
realtime/documentation/howto/tools/perf-smi-cost.txt ยท Last modified: 2018/08/21 15:41 by ebugden
Page Tools
Except where otherwise noted, content on this wiki is licensed under the following license: CC Attribution-Noncommercial-Share Alike 4.0 International
