15.1 Measuring Performance

The system-perfmeter extension can be used to understand the performance you get out of Simics. The system-perfmeter is sample based, which means that you can see the performance during the workload execution, and how it varies, not only the end result when a workload is finished.

The easiest way to try it out is simply to issue the system-perfmeter command without any additional arguments:

simics> system-perfmeter

This will cause a sample to be taken every 1.0 virtual seconds. For each sample the system-perfmeter extracts various counters from Simics and displays the delta since last time. The output can look like this:

simics> c
SystemPerf: Total-vt Total-rt Sample-vt Sample-rt Slowdown  CPU Idle
SystemPerf: -------- -------- --------- --------- -------- ---- ----
SystemPerf:     1.0s     6.0s     1.00s     5.99s     5.99  97%   0%
SystemPerf:     2.0s     6.7s     1.00s     0.69s     0.69  97%   0%
SystemPerf:     3.0s     8.0s     1.00s     1.34s     1.34  92%   0%
SystemPerf:     4.0s     8.4s     1.00s     0.42s     0.42 100%   0%
SystemPerf:     5.0s     9.2s     1.00s     0.78s     0.78  98%  14%
SystemPerf:     6.0s    10.5s     1.00s     1.31s     1.31  96%  55%
SystemPerf:     7.0s    10.7s     1.00s     0.12s     0.12  92%  93%
SystemPerf:     8.0s    10.7s     1.00s     0.00s     0.00 100% 100%

Here we can see the execution for the first 8 virtual seconds and the corresponding performance measured in each second sample. To simulate these 8 virtual seconds, it took Simics 10.7 host seconds, thus the average slowdown is 1.34.

The CPU column shows how much of the host CPU that Simics has used, allowing you to notice if there is another process consuming the host CPU resources. Another reason for CPU utilization to be low can be that Simics itself is running in real-time mode where Simics sleeps so that virtual time does not race ahead of host time.

When an 'idle' condition has been detected (see chapter 15.4.1), the total idleness of the system is reported in the Idle column. If the simulated system consists of multiple processors and you wish to see how much each processor is idling you can use the -cpu-idle switch to system-perfmeter. Note that idling is defined by the simulator, not by the target architecture (see chapter 15.4.1). With the -cpu-exec-mode, information is also gathered and printed on how simulation steps are executed in the CPU model. The fastest mode to be executing in is idle, followed by VMP, JIT, and interpreter.

The system-perfmeter can also be used to get an understanding of which processor that takes the longest time to simulate. The -cpu-host-ticks switch adds extra columns per CPU for this. For example:

SystemPerf: Total-vt Total-rt Sample-vt Sample-rt Slowdown  CPU Idle [   0   1   2   3 ]
SystemPerf: -------- -------- --------- --------- -------- ---- ----   --- --- --- ---
SystemPerf:     1.0s    15.6s     1.00s    15.63s     15.6  99%  74% [  71  11  11   8 ]
SystemPerf:     2.0s    32.6s     1.00s    16.97s     17.0  98%  72% [  70  11  10   9 ]

Here we have a 4 CPU system which is idle roughly 70% and the last columns show that CPU0 takes 70% of the time to simulate, while the other three about 10% each. Hence CPU0 is working while the other CPUs are idling.

The -module-profile flag enables live profiling of the main Simics thread. The profiling is sample based, and any sample hitting in code produced dynamically by a JIT engine will be reported as "classname JIT". The -module-profile data is not printed in the standard line print mode, so you must use either -top or -summary to get profiling information.

Figure 30. Annotated output from system-perfmeter -top -module-profile -cpu-exec-mode

Figure 30 explains the output of system-perfmeter. There are many other options to the system-perfmeter command, please read the associated help text for details.

simics> help system-perfmeter