Jim - 8 months ago 56

C# Question

I am trying to throttle a loop (which is sending messages) to a particular number of messages per second.

`_throttle`

My initial algorithm is depicted below, but the delays are not smooth.

I have played around with the tick, and the interval max, but the inbound count is so large it's hard to compensate. The maximum rate I can achieve with the throttle off in my implementation is about 15000/second. I am testing with rates between 300 and 1000 per second, so I am trying to slow it down quite a bit.

`private class ThrottleCalculator`

{

private readonly int _throttle;

private DateTime _lastCalculation = DateTime.Now;

private int _count = 0;

private int _interval = 0;

public ThrottleCalculator(int throttle)

{

this._throttle = throttle;

}

public async Task CalculateThrottle()

{

this._count += 1;

var elapsed = DateTime.Now.Subtract(this._lastCalculation).TotalMilliseconds;

var tick = 50;

if (elapsed > tick)

{

this._lastCalculation = DateTime.Now;

int projection = this._count * (1000 / tick);

var errorTerm = this._throttle - projection;

this._interval = this._interval - errorTerm;

if (this._interval < 0)

this._interval = 0;

// this is often several thousand, so I have to limit.

if (this._interval > 100)

this._interval = 100;

await Task.Delay(this._interval);

this._count = 0;

}

}

}

The code that uses this just calls this every iteration.

`var throttle = new ThrottleCalculator(600); // 600/s`

while (message = getMessage())

{

... // do stuff with message.

if (throttle != null)

await throttle.CalculateThrottle();

Answer

for anyone else attempting this, the correct approach is the **PID controller algorithm.**

`Proportional / Integral / Derivative Controller`

I used the algorithm at the bottom of the wiki as a base.
My `kp / ki / kd`

seem to work well with the values here, keeping them in proportion seems to result in a nice steady stream of messages, and very tight delay values.

```
private class ThrottleCalculator
{
private readonly int _throttle;
private DateTime _lastCalculationTime;
private double _measured = 0;
private double _totalError = 0;
private double _integral = 0;
private double _lastError = 0;
public ThrottleCalculator(int throttle)
{
this._throttle = throttle;
this._lastCalculationTime = DateTime.MinValue;
}
public async Task CalculateThrottle()
{
var kp = -.1d; // proportional gain
var ki = -.1d; // integral gain
var kd = -.1d; // derivative gain
var dt = 30d; // rate of change of time. calculcations every ms;
this._measured += 1;
if (this._lastCalculationTime == DateTime.MinValue)
this._lastCalculationTime = DateTime.Now;
var elapsed = (double)DateTime.Now.Subtract(this._lastCalculationTime)
.TotalMilliseconds;
if (elapsed > dt)
{
this._lastCalculationTime = DateTime.Now;
var error = ((double)this._throttle / (1000d / dt)) - this._measured;
this._totalError += error;
var integral = this._totalError;
var derivative = (error - this._lastError) / elapsed;
var actual = (kp * error) + (ki * integral) + (kd * derivative);
var output = actual;
if (output < 1)
output = 0;
// i don't like this, but it seems necessary
// so that wild wait values are never used.
if (output > dt * 4)
output = dt * 4;
if (output > 0)
await Task.Delay((int)output);
this._measured = 0;
this._lastError = error;
}
}
}
```

My values look like this:

```
Actual: 19.2000 Output: 19.2000 Integral: -209 Derivative: .0000 Error: 17
Actual: 17.5000 Output: 17.5000 Integral: -192 Derivative: .0000 Error: 17
Actual: 15.8000 Output: 15.8000 Integral: -175 Derivative: .0000 Error: 17
Actual: 33.8104 Output: 33.8104 Integral: -255 Derivative: -3.1040 Error: -80
Actual: 21.8931 Output: 21.8931 Integral: -238 Derivative: 2.0686 Error: 17
Actual: 20.4000 Output: 20.4000 Integral: -221 Derivative: .0000 Error: 17
Actual: 18.7000 Output: 18.7000 Integral: -204 Derivative: .0000 Error: 17
Actual: 17.0000 Output: 17.0000 Integral: -187 Derivative: .0000 Error: 17
Actual: 15.3000 Output: 15.3000 Integral: -170 Derivative: .0000 Error: 17
Actual: 31.0752 Output: 31.0752 Integral: -239 Derivative: -2.7520 Error: -69
```