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CPU Threading Efficiency: New Performance Tuning Perspectives

 CPU threading

My point: While a virtual CPU is being served by a CPUcore, its logical CPUs are the realization of the dynamic SMT mode of the CPUcore. In other words, the existence/productivity of a virtual CPU’s logical CPUs is the manifestation of the dynamic SMT mode of its CPUcore. In Figure 2, virtual CPU:Proc0 is comprised of logical CPUs:cpu0|cpu1|cpu2|cpu3, virtual CPU:Proc4 is comprised of logical CPUs:cpu4|cpu5|cpu6|cpu7, etc.

Again, the dynamic SMT mode underlies the calculation of CPU idle%.

The Role of AIX:wait Kernel Processes

AIX:wait kernel processes are unnoticed by most AIX administrators. Without doubt, all monitoring utilities have also ignored these processes. As such, they have historically been considered unimportant.

But what if I told you that AIX:wait kernel processes account for 20-80 percent of all CPU processing in your LPARs? Would they then be worthy of attention?

Figure 3 uses AIX:ps guww to list the AIX:wait kernel processes. Each AIX:wait process is bound and dedicated to one and only one logical CPU upon boot. An obscure variation of the AIX:ps (process status) command found in UNIX/AIX, the AIX:ps guww syntax in figure 3 displays the substantial amounts of on-CPU time of multiple AIX:wait kernel processes and multiple db2sysc (DB2 database) processes under the TIME column (second from the far right).

The TIME column reports the total on-CPU duration of the process in minutes:seconds since the process’s STIME time stamp (third from the far right). STIME is the clock time or calendar date the process was created. All of the listed processes in figure 3 were started on Dec 06.

Compare the accumulated on-CPU minutes:seconds of the TIME column values for the wait and db2sys processes.

Have I established my point that AIX:wait kernel processes are worthy of attention?

If you guess that AIX:wait kernel processes do as they are named, you are correct. AIX:wait kernel processes do nothing but hold and wait―that is, the dynamic SMT mode is merely holding a logical CPU in place while waiting for a workload thread to execute. Maintaining some measure of running AIX:wait processes ensures a workload thread has an available logical CPU for immediate attention (an aspect of thread responsiveness). This is a valid and vital function. Unfortunately, when CPUcores are wastefully executing too much/too many AIX:wait processes, no other virtual CPU (of this or any other LPAR) can use these same CPUcores for more productive processing. The trick is to configure just enough AIX:wait processes to address thread responsiveness, and not too much/too many.

For another perspective, AIX:wait processes accumulate (and thus account) the time that available logical CPUs have nothing to do (while waiting for a workload thread to execute). Traditionally, CPUs with nothing to do were considered idle CPU cycles. However, on Power/AIX, CPUs are never unaccountably idle because the idle CPU cycles of their logical CPUs are accounted by AIX:wait processes. What was historically considered idle CPU cycles are accounted here as an accumulation of CPU time executing AIX:wait processes. In contrast, the meaning and calculation of CPU idle% is an entirely different concept.

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