9.2 Probe Monitor

The probe-monitor tool offers a way to read out and present the probe values in continuous samples, while the simulation is running.

All probes being monitored will be presented in a table row for each sample taken, allowing detection of abnormal values during the simulation.

It is possible to add lots of probes to the same probe-monitor, but as the number of probes increase, so does the width of the produced table. Therefore, it can be useful to create several probe-monitors, each showing some group of related feature.

To create a probe-monitor use the new-probe-monitor command. This will create a new object such as pm0 which has some additional commands. See figure 25 for an example.

$> ./simics targets/qsp-x86/firststeps.simics num_cores=2
simics> new-probe-monitor mode = realtime interval=1 -summary      
simics> pm0.add-probe probe = sim.time.wallclock sim.time.virtual mode = session
simics> pm0.add-probe probe = sim.time.schedule sim.slowdown sim.esteps sim.mips cpu.esteps cpu.time.schedule cpu.schedule_percent cpu.mips
simics> run-seconds 60
┌──────────────────────────────────────────────────────────────┬──────────────────────────────────────┬──────────────────────────────────────┐
│                                                              │       board.mb.cpu0.core[0][0]       │       board.mb.cpu0.core[1][0]       │
├──────────┬──────────┬─────────┬────────┬────────────┬────────┼────────────┬─────────┬──────┬────────┼────────────┬─────────┬──────┬────────┤
│ Session  │ Session  │Sched (s)│Slowdown│   Esteps   │  IPS   │   Esteps   │Sched (s)│Sched%│  IPS   │   Esteps   │Sched (s)│Sched%│  IPS   │
│Wallclock │ Virtual- │         │        │            │        │            │         │      │        │            │         │      │        │
│(hh:mm:ss)│   Time   │         │        │            │        │            │         │      │        │            │         │      │        │
│          │(hh:mm:ss)│         │        │            │        │            │         │      │        │            │         │      │        │
├──────────┼──────────┼─────────┼────────┼────────────┼────────┼────────────┼─────────┼──────┼────────┼────────────┼─────────┼──────┼────────┤
│  00:00:01│  00:00:01│378.85 ms│    0.99│   696409015│739.61 M│   531985315│317.16 ms│   84%│  1.68 G│   164423700│ 61.69 ms│   16%│  2.67 G│
│  00:00:02│  00:00:10│997.38 ms│    0.11│  1158080848│  1.15 G│   714229158│840.88 ms│   84%│849.38 M│   443851690│156.49 ms│   16%│  2.84 G│
│  00:00:03│  00:00:30│998.06 ms│    0.05│    71030255│ 70.95 M│    56546115│949.91 ms│   95%│ 59.53 M│    14484140│ 48.15 ms│    5%│300.84 M│
│  00:00:04│  00:00:49│997.34 ms│    0.05│    69869598│ 69.78 M│    56185665│949.43 ms│   95%│ 59.18 M│    13683933│ 47.91 ms│    5%│285.62 M│
│  00:00:05│  00:00:56│997.45 ms│    0.16│  3271646626│  3.27 G│  3244559622│967.68 ms│   97%│  3.35 G│    27087004│ 29.77 ms│    3%│910.00 M│
│  00:00:06│  00:00:57│997.85 ms│    0.82│   919200160│917.96 M│   653789980│712.32 ms│   71%│917.83 M│   265410180│285.53 ms│   29%│929.55 M│
│  00:00:07│  00:00:59│998.80 ms│    0.41│   603301058│603.17 M│   305625523│566.57 ms│   57%│539.43 M│   297675535│432.24 ms│   43%│688.69 M│
│  00:00:08│  00:00:60│999.04 ms│    1.51│  2209183348│  2.21 G│  1128067814│442.09 ms│   44%│  2.55 G│  1081115534│556.95 ms│   56%│  1.94 G│

Here, first a probe-monitor is created, that gets the name pm0 by default. The probe-monitor samples the probes each wallclock seconds, which is controlled by the mode and interval arguments.

The -summary flag means all probe values will be displayed when the simulation stops, providing the session or current value for the probes subscribed to, described later.

Next, the pm0.add-probe is issued with two probes using the mode session. If mode is not specified, it defaults to delta meaning the value presented for the probe is the difference from the previous sample. That is, it shows the value for the sample only. With the session mode, it instead shows the total value since the probe was added to the monitor. In this case, the accumulated times, both in host wall-clock and the virtual time of the simulated system is displayed.

Apart from session and delta there is also the current mode which can be selected, which displays the plain probe value, without any adjustments. The current mode is interesting for some probes, which are not monotonically increasing, such as a temperature. It can also be interesting to be used for probes derived from checkpointed state, for example the global virtual time, when the user wants to see the absolute time.

Finally, before the execution is started we add more probes, now using the default delta mode. Here we are mixing some global probes sim.* and some processor specific probes cpu.*. Since there are two processors in the simulated system used in this example, the number of all monitored probes is 14.

When the simulation is started, the probe-monitor starts to print out table rows for the probes being measured, one second between each sample. Probes which have an object associated will be sorted under that object, so here probes for the two processors have an additional heading showing where the probes belong to.

Some more detailed information on each column or probe, that was monitored in this example.

Session Wallclock (hh:mm:ss) - sim.time.wallclock

This is basically the host time that has elapsed during the simulation. Note that this time stands still if Simics is not executing. If the true wallclock is desired (which increments even when Simics is not simulating), the probe host.time.wallclock can be used instead. The time here is presented as hours, minutes and second format.

Session Virtual-Time (hh:mm:ss) - sim.time.virtual

This probe returns the virtual time of the simulation. The virtual time for all processors are pretty much in sync, but there can be small differences. The probe simply selects the first processor that it finds and presents that. Similar to wall clock, and all session or current times are presented in hours, minutes and second format.

Sched (s) - sim.time.schedule

This is another aspect of time in Simics, namely how much time that Simics scheduler have requested the different processor to be scheduled in total. In a multi-threaded simulation this time can be much greater than the wallclock time, since each execution thread can run in parallel. The monitor presents any delta times with a second metric-prefix, giving much higher resolution.

Slowdown - sim.slowdown

The slowdown probes gives an measurement on how fast virtual time elapses compared to wallclock time. Any value below one means that the virtual time goes faster then the host wallclock. This provides a hint on how fast the simulation is.

A value of 0.5 means Simics simulation goes twice as fast as the simulated virtual system, while a figure of 2.0 means the simulation goes in half the speed compared to the simulated system.

Note: Don't take this figure for any kind of exactness to the real system, sim.time.virtual depends on:

• The frequency of the simulated processors.
• A simplistic timing models where typically one instruction always takes one cycle to execute.
• Device models are also typically simplistic on how fast transfers and interrupts occur.
• Idle processor can typically consume cycles almost infinitely fast, which does not happen in real hardware.

How fast Simics executes also depends on how many processors it schedules, the processor models, the type of workload, threading settings, internal caches and much more. Thus, one needs to take the slowdown figure with a grain of salt, before comparing with other sessions, or even with just with different workloads.

Esteps - sim.esteps

Steps in Simics are roughly equivalent to instructions. Some processor models however can consume steps while they are stalled. Therefore, the esteps (executed steps) metric is better suited for counting instructions, where the halt-steps have been subtracted from the steps counter. The simulator global sim.esteps is the total of all cpu.esteps in the system.

IPS - sim.mips

(M)IPS is an acronym of (Million) Instructions Per Second. This is a common measurement for looking at the how fast the simulation is. It is calculated as sim.esteps / sim.time.wallclock. Note that, depending on the simulation speed, other prefixes then M is used each sample, for example G for GIPS, or k for kIPS.

Esteps - cpu.esteps

The processor specific number of executed steps.

Sched (s) - cpu.time.schedule

The amount of time Simics scheduler has scheduled this processor.

Sched% - cpu.schedule_percent

This probe gives a percent value of how much of the time this processor has been scheduled cpu.time.schedule / sim.time.schedule This can be used to detect bottle-necks in the simulation, if one processor stands for the majority of the time.

IPS - cpu.mips

This probe might look like sim.mips but is actually a bit different. The processor specific MIPS number is calculated as: cpu.esteps / cpu.time.schedule. This means that the MIPS numbers here cannot be added together. Instead this gives an indication of how fast this particular processor executes, given the amount of time it got. This can be used to detect sub-optimal processor simulation even if the processor runs at a much lower frequency, and consequently is not scheduled as much.

When the simulation is finished, the probe-monitor shows a table of all the probes that are monitored. The table provides the current value for all probes, see figure 26 for the example output.

┌─────┬────────────┬────────────────────┬────────────────────────┬───────────────────────┐
│Row #│Display Name│     Probe Name     │         Object         │Session Formatted Value│
├─────┼────────────┼────────────────────┼────────────────────────┼───────────────────────┤
│    1│Wallclock   │sim.time.wallclock  │Global                  │            00:00:08.16│
│    2│Virtual-Time│sim.time.virtual    │Global                  │            00:01:00.00│
│    3│Sched       │sim.time.schedule   │Global                  │            00:00:07.57│
│    4│Slowdown    │sim.slowdown        │Global                  │                   0.14│
│    5│Esteps      │sim.esteps          │Global                  │             9550122021│
│    6│IPS         │sim.mips            │Global                  │                 1.17 G│
│    7│Esteps      │cpu.esteps          │board.mb.cpu0.core[0][0]│             6959267798│
│    8│Sched       │cpu.time.schedule   │board.mb.cpu0.core[0][0]│            00:00:05.88│
│    9│Sched%      │cpu.schedule_percent│board.mb.cpu0.core[0][0]│                 77.73%│
│   10│IPS         │cpu.mips            │board.mb.cpu0.core[0][0]│                 1.18 G│
│   11│Esteps      │cpu.esteps          │board.mb.cpu0.core[1][0]│             2590854223│
│   12│Sched       │cpu.time.schedule   │board.mb.cpu0.core[1][0]│            00:00:01.69│
│   13│Sched%      │cpu.schedule_percent│board.mb.cpu0.core[1][0]│                 22.27%│
│   14│IPS         │cpu.mips            │board.mb.cpu0.core[1][0]│                 1.54 G│
└─────┴────────────┴────────────────────┴────────────────────────┴───────────────────────┘

The Session Formatted Value represents the probe value formatted accordingly to the probe properties. If probes have been added with the current mode then an additional Current Formatted Value column is also displayed.

It is also possible to get the same results printed out with the pm0.summary command, where it is possible to request more floating point decimals to be presented with the float-decimals argument.

The pm0.status shows the settings of the probe-monitor, including which probes that are currently monitored. Probes can be removed with the pm0.remove-probe command and it is possible to change the sampling settings of the probe-monitor itself with the pm0.sampling-settings command.

While the probe-monitor prints out the table data while the simulation is running, all data is also saved internally. To show the entire history of all samples, the pm0.print-table can be used and the data can also be saved into a .csv with the pm0.export-table-csv command. Any data in the csv file will be unformatted, allowing easily import in a spreadsheet for further analyses.