Multiple-process Bottlenecks

In a bottleneck caused by a multiple processes, no single process stands out above all others. When multiple processes are involved, several might be active, each using a smaller proportion of processor cycles. Multiprocess bottlenecks usually result when the processor cannot handle the process load. They do not usually indicate a problem with an application.

The following figure shows a histogram of processor time for many active processes.

This example was produced by using four copies of a simulation tool, CPU Stress, which consumes processor cycles at a priority and activity level you specify.

In this example, the highest bar on the far left is System: % Total Processor Time. At least four other processes (represented by bars reaching about 20% processor time) are consuming the processor while sharing it nearly equally. Although each process is only using 10–20% of the processor, the result is the same as a single process using 100% of processor time.

The following figure shows Processor: Processor Queue length during this bottleneck.

In the graph, Processor: % Processor Time (the black line running across the top of the graph) remains at 100% during the sample interval. System: Processor Queue Length (the white line) reveals a long queue. The value bar shows that the queue length varies between 6 threads and 12 threads, and averages over 7.5 threads.

The following figure shows Task Manager during the same bottleneck. It shows that four copies of CPU Stress are each using about one-fifth of the time of the single processor on the computer. (Task Manager displays current values, so you need to watch the display to see changes in processor use for each process.)

Although a faster processor might help this situation somewhat, multiple-process bottlenecks are best resolved by adding another processor. Multithreaded processes, including multithreaded Windows NT services, benefit the most from additional processors because their threads can run simultaneously on multiple processors. Even after adding another processor, it is prudent to continue testing with different priorities and processor loads to resolve this more complex situation.