PiDP-8/I Software

Throttling the Simulator
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When you do not give the --throttle option to the configure script, the simulator's speed is set based on the number of CPU cores detected by the tools/corecount script.

Multi-Core Default

If corecount detects a multi-core system, the default behavior is to not throttle the simulator at all, since there are only 2 threads in the software that take substantial amounts of CPU power.

The most hungry thread is the PDP-8 simulator proper, which runs flat-out, taking an entire core's available power by default.

The other hungry thread is the one that drives the front panel LEDs, which takes about 35% of a single core's power on a Raspberry Pi 3 when you build the software with the incandescent lamp simulator enabled.

This leaves over 2 cores worth of CPU power untapped on multi-core Raspberry Pis, so the system performance remains snappy even with the simulator running.

You can force this behavior with --throttle=none.

Single-Core Default

If the configure script decides that you're building this on a single-core system, it purposely throttles the PDP-8 simulator so that it takes about 50% of a single core's worth of power on your system.

Since different single-core Raspberry Pi boards run at different clock rates, we cannot just set the instructions per second (IPS) rate to a fixed value and know that it's correct.

We could choose a value low enough that it's supposed to work everywhere, but there's a serious problem with that. SIMH's reaction to not having enough host CPU power to run at the requested IPS rate is to turn off throttling entirely, thus hogging the whole host CPU, exactly the opposite of what you want by turning on throttling! Host CPU power is sapped during boot time, the point where the PiDP-8/I software normally starts up. That badly throws off SIMH's estimates of the hardware's capability, do you might get thrown into no-throttle mode even if the hardware could actually achieve the requested IPS rate once it has the CPU mostly to itself.

Our chosen solution to all of these problems is set throttle 50%, which tells SIMH to dynamically adjust the IPS rate according to available host CPU power. This does mean you don't get a steady IPS rate, but it does enforce our wish to leave some CPU power left over for background tasks.

(There's an alternate fix for this in recent versions.)

You might wish to adjust that default host/simulator CPU usage split. The --throttle option accepts percentages:

 ./configure --throttle=75%

This throttling feature does not allow a single-core Pi to run the incandescent lamp simulator. We've tried, and you have to clock the simulator down to less than the speed of a PDP-8/S to leave enough CPU power for the ILS to run properly! (DEC says the "S" refers to the serial bit handling path in this cost-reduced version of the original PDP-8, but everyone else knows the "S" stands for SSSLLLOOOOWWW.) We have ideas for ways to improve the speed of the ILS which may allow the PDP-8 simulator to run at a reasonable rate alongside the ILS, but these plans may never make their way off the wish list, or they may not succeed if they are implemented. For this reason, the build system disables the incandescent lamp simulator feature on a single-core Pi.

You can force the build system to select this throttle value even on a multi-core Pi with --throttle=single-core.

You will erroneously get this single-core behavior if you run the configure script on a system where tools/corecount has no built-in way to count the CPU cores in your system correctly, so it returns 1, forcing a single-core build. That script currently only returns the correct value on Linux, BSD, and macOS systems. To fix it, you can either say --throttle=none or you can patch tools/corecount to properly report the number of cores on your system. If you choose the latter path, please send the patch to the mailing list so it can be integrated into the next release of the software.


There are many reasons to make the software run slower than it normally would. You may achieve such ends by giving the --throttle option to the configure script:

Throttle Stabilization

After the 2017.12.22 release, the upstream SIMH v4 project's main developer changed the way throttling is handled, mostly for the better. The primary upshot of this change — from the perspective of this document's subject matter — is that the simulator doesn't make any decisions about whether your requested throttle value is plausible until some seconds after the simulator starts.

The SIMH default for this is 20 seconds, since the default must work for all simulators in the SIMH family, some of which have long bootup cycles. 20 seconds is far too long for a PDP-8, which boots instantly, so we've overridden that in the stock boot/*.script files, setting the throttle calibration delay to 3 seconds in order to give the SIMH timing code a sufficiently long baseline to work from.

If you have a prior installation of the PiDP-8/I software, your boot scripts will not have this change unless you have taken specific steps to achieve it, so you will be using the 20-second SIMH default! See "Overwriting the Local Simulator Setup" in the top-level file for your options here.

For those first seconds, the simulator runs unthrottled, after which the SIMH core timing code looks at the number of instructions executed during that time and then determines from that what timing values it needs to use to achieve your requested throttle value. It also checks whether the throttle value is even possible at this time.

There is one case where we anticipate that you might want to increase this value: you've set a fixed throttle value that is right near the host CPU's ability to achieve and you have the simulator set to start at host boot time, so that there are lots of processes starting up and initializing in parallel with the PiDP-8/I simulator. In that case, you might want to override the following line in boot/*.script, setting a long enough value for the system load to stabilize:

deposit int-throttle THROT_DELAY 15

That would override the 20-second stabilization time default to 15 seconds.

I/O Matters

The throttle mechanism discussed above only affects the speed of the PDP-8 CPU simulator. It does not affect the speed of I/O operations.

The only I/O channel you can throttle in the same way is a serial terminal by purposely choosing a slower bit rate for it than the maximum value. If you set it to 110 bps, it runs at the speed of a Teletype Model 33 ASR, the most common terminal type used for the PDP-8/I, and most other early PDP-8 flavors. Later in the PDP-8's commercial life, faster glass terminals became available, though it was not always possible to use them at their maximum data rate, due to the primitive nature of I/O handling in those days.

I'm not aware of a way to make SIMH slow the other I/O operations, such as disk access speeds, in order to emulate the speed of the actual hardware.


Copyright © 2017-2018 by Warren Young. This document is licensed under the terms of the SIMH license.