xcal: Processor cross-calls (when one CPU wakes up
another by interrupting it). If this exceeds 200/second, the application
in question may need to be examined.
intr: Interrupts.
ithr: Interrupts as threads (except clock).
csw: Context switches
icsw: Involuntary context switches (this is probably
the more relevant statistic when examining performance issues.)
migr: Thread migrations to another processor.
If the migr measurement of mpstat is greater
than 500, rechoose_interval should be sent longer in the
kernel.
smtx: Number of times a CPU failed to obtain a mutex.
srw: Number of times a CPU failed to obtain a read/write
lock on the first try.
syscl: Number of system calls.
usr/sys/wt/idl: User/system/wait/idle CPU percentages.
