Add what-is-skid.md

SUMMARY.md

@@ -3,6 +3,7 @@
 * [An example of profiling application](posts/an-example-of-profiling-application.md)
 * [The targets of profiling](posts/the-targets-of-profiling.md)
 * [How to specify monitoring events](posts/how-to-specify-monitoring-events.md)
+* [What is skid](posts/what-is-skid.md)
 * [Count events](posts/count-events.md)
 * [Profile memory access](posts/profile-memory-access.md)
 * [Check cache false sharing](posts/check-cache-false-sharing.md)

posts/what-is-skid.md

@@ -0,0 +1,21 @@
+# What is skid
+
+Skid is the distance between events which trigger the issue and the events which are actually caught by `perf`. To improve sampling accuracy, `Intel` brings in `PEBS (Precise Event-Based Sampling)`, whilst `AMD` introduces `IBS (Instruction-Based Sampling)`. Check the output of "`perf list --help`":  
+
+	# perf list --help
+	......
+	The p modifier can be used for specifying how precise the instruction address should be. The p modifier can be specified multiple times:
+
+           0 - SAMPLE_IP can have arbitrary skid
+           1 - SAMPLE_IP must have constant skid
+           2 - SAMPLE_IP requested to have 0 skid
+           3 - SAMPLE_IP must have 0 skid, or uses randomization to avoid
+               sample shadowing effects.
+
+	For Intel systems precise event sampling is implemented with PEBS which supports up to precise-level 2, and precise level 3 for some special cases
+
+    On AMD systems it is implemented using IBS (up to precise-level 2). 
+
+Following command is an example which requires no skid for sampling `mem-loads` event.
+
+	# perf record -e "cpu/mem-loads/pp"