Difference between revisions of "Performance optimization"

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SPI.setClockDivider(SPI_CLOCK_DIV2) //runs the SPI at maximum speed (8Mhz)
 
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Revision as of 2013-10-30T20:49:46

Introduction

The following apply to Gamebuino, Arduino and any 8-bit microcontroller as well.

Gamebuino is a video game console which has limited hardware (8-bits CPU running at 16Mhz and 2KB of RAM...). It's enough if you want to program Pong or Tetris, but if you plan to do some advanced games with multiplayer and artificial intelligence, you will probably have to optimize your game for it to run fast enough and fit in the chip's memory.

Most of the optimization work is done at high level, by designing smart algorithms. But sometimes it's not enough, and to go further you have to get into more system-specific programming to gain speed and memory.

Premature optimization

Don't optimize something that doesn't need to be. First write a clean code, something easy to read and maintain. Then, only if you actually need performance improvement, benchmark your code to know what is most time consuming and likely to be optimized. Don't try to guess what should be optimized, because you'll be wrong most of the time, and you'll spend time optimizing something that isn't the bottleneck.

Don't be smart

"You should think REALLY, REALLY hard about sacrificing code clarity for the sake of speed or space." westfw on Arduino forum

Variables

data types

You should always use the smallest variable type you need. Because Gamebuino runs on a 8-bits microcontroller, 16 and 32 bits variables takes a lot of time and memory, and floating point variables are even worse than integers.

Most of the time you can re-arrange math to avoid the need of floating point. For example, the battery voltage is stored in mV instead of V, so we can use a unsigned int instead of a floating point.

Arduino data types
Type Min Max Size Perf
byte 0 255 8  :)
char -128 127 8  :)
unsigned int 0 65 535 16  :/
int −32 768 32 767 16  :/
unsigned long 0 4,294,967,295 32  :(
long -2,147,483,648 2,147,483,647 32  :(
float / double -3.4028235E+38 3.4028235E+38 32  :'(

For more information about the data type refer to Arduino Reference.

#define

You can use #define for constant variables that will never change, like pin numbers. This way it won't take any space in the chip's memory.

Here is an example :

#define ledPin 13

progmem

For constant large arrays or bitmaps you can use progmem to avoid loading them in RAM. Arduino article explains that, for more info read a complete tutorial.

Operations & Math

Arithmetic operators

Addition is the fastest, multiplication is slower, and division is even slower.

To multiply or divide by a power of 2 you can use bitwise operators << and >> but remember, "Don't be smart".

Math

Avoid using sin() cos() and these kind of operations, because they are very slow and take a lot of memory. Moreover, they manipulate floating point variables, which are also time and memory consuming.

Loops

Communication

Serial

To increase Gamebuino's / Arduino's USB communication speed, you can increase the baud rate (number of pulse per second). Usually 9600 is the default value, but you can use 115200 without problem. To set the baudrate, just call this function at the beginning of your setup() function :

Serial.begin(115200);

Warning : the baud rate of the PC have to be the same or you will end receiving hieroglyphs.

Note : Starting from Arduino 1.0, serial is no longer blocking. Data will be sent in background using interruptions. To waits for the transmission of outgoing serial data to complete, use Serial.flush().

SPI

You can increase the SPI speed by changing the clock divider using SPISetClockDivider.

SPI.setClockDivider(SPI_CLOCK_DIV2) //runs the SPI at maximum speed (8Mhz)

Warning : On Gamebuino, screen's specified maximum SPI frequency is 4Mhz, so if you use a clock divider < 4 it will act erratically.

I2C / TWI

Clock frequency

Arduino vs native AVR

Assembly language

Other tips & tricks

Further reading