I am collecting data from a USB device and this data has to go to an audio output component. At the moment I am not delivering the data fast enough to avoid clicks in the output signal. So every millisecond counts.
At the moment I am collecting the data which is delivered in a byte array of 65536 bytes.The first two bytes represent 16 bits of data in little endian format. These two bytes must be placed in the first element of a double array. The second two bytes, must be placed in the first element of a different double array. This is then repeated for all the bytes in the 65536 buffer so that you end up with 2 double[] arrays of size 16384.
I am currently using BitConverter.ToInt16 as shown in the code. It takes around 0.3ms to run this but it has to be done 10 times to get a packet to send off to the audio output. So the overhead is 3ms which is just enough for some packets to not be delivered on time eventually.
Code
byte[] buffer = new byte[65536];
double[] bufferA = new double[16384];
double[] bufferB = new double[16384]
for(int i= 0; i < 65536; i +=4)
{
bufferA[i/4] = BitConverter.ToInt16(buffer, i);
bufferB[i/4] = BitConverter.ToInt16(buffer, i+2);
}
How can I improve this? Is it possible to copy the values with unsafe code? I have no experience in that. Thanks
This gets me about triple the speed in release, using Pointers and unsafe. There maybe other micro-optimisations, however I'll leave those details up to the masses
My original algorithm had a bug, and could have been improved
Modified Code
public unsafe (double[], double[]) Test2(byte[] input, int scale)
{
var bufferA = new double[input.Length / 4];
var bufferB = new double[input.Length / 4];
fixed (byte* pSource = input)
fixed (double* pBufferA = bufferA, pBufferB = bufferB)
{
var pLen = pSource + input.Length;
double* pA = pBufferA, pB = pBufferB;
for (var pS = pSource; pS < pLen; pS += 4, pA++, pB++)
{
*pA = *(short*)pS;
*pB = *(short*)(pS + 2);
}
}
return (bufferA, bufferB);
}
Each test is run 1000 times, garbage collected before each run, and scaled to various array lengths. All results are checked against the original OP version
Test Environment
----------------------------------------------------------------------------
Mode : Release (64Bit)
Test Framework : .NET Framework 4.7.1 (CLR 4.0.30319.42000)
----------------------------------------------------------------------------
Operating System : Microsoft Windows 10 Pro
Version : 10.0.17134
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CPU Name : Intel(R) Core(TM) i7-3770K CPU @ 3.50GHz
Description : Intel64 Family 6 Model 58 Stepping 9
Cores (Threads) : 4 (8) : Architecture : x64
Clock Speed : 3901 MHz : Bus Speed : 100 MHz
L2Cache : 1 MB : L3Cache : 8 MB
----------------------------------------------------------------------------
Results
--- Random Set of byte ------------------------------------------------------
| Value | Average | Fastest | Cycles | Garbage | Test | Gain |
--- Scale 16,384 -------------------------------------------- Time 13.727 ---
| Unsafe | 19.487 µs | 14.029 µs | 71.479 K | 0.000 B | Pass | 59.02 % |
| Original | 47.556 µs | 34.781 µs | 169.580 K | 0.000 B | Base | 0.00 % |
--- Scale 32,768 -------------------------------------------- Time 14.809 ---
| Unsafe | 40.398 µs | 31.274 µs | 145.024 K | 0.000 B | Pass | 56.62 % |
| Original | 93.127 µs | 79.501 µs | 329.320 K | 0.000 B | Base | 0.00 % |
--- Scale 65,536 -------------------------------------------- Time 18.984 ---
| Unsafe | 68.318 µs | 43.550 µs | 245.083 K | 0.000 B | Pass | 68.34 % |
| Original | 215.758 µs | 160.171 µs | 758.955 K | 0.000 B | Base | 0.00 % |
--- Scale 131,072 ------------------------------------------- Time 22.620 ---
| Unsafe | 120.764 µs | 79.208 µs | 428.626 K | 0.000 B | Pass | 71.24 % |
| Original | 419.889 µs | 322.388 µs | 1.461 M | 0.000 B | Base | 0.00 % |
-----------------------------------------------------------------------------