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Programming
in the small
Henk Muller
Principal Technologist, XMOS
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XMOS
- A company that designs and sells microprocessors.
- We design a processor (using Verilog - You may have used that for Computer Architecture)
- We ship the design to a manufacturing plant
- We get the chips back
- We sell the chips.
Challenge:
- Make a good processor (“better” than everybody else’s)
- Make a good software development platform (“better” again)
- Make good application software.
We do not sell software, but half the company staff are designing software of one sort or another...
Henk MullerASD lecture, November 24, 2010 (^2)
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Tools for internal use
We need a tool chain to build the processor and the tools
- Precise simulator
- Emulate the processor in software.
- Build systems
- Regression systems
- Build, run and compare everything against expected output.
- Verification systems
- Mechanically compare two implementations against each other
- Hardware timing and power analysis tools
- Tools that tell us how the chip will perform.
Henk MullerASD lecture, November 24, 2010 (^4)
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Tools for customers
The processor needs a tool chain for software development
- Compilers
- Linker
- Debugger
- Timing analyser
- IDE
Additional tools required for
- Downloading code
- Burning code onto the device
- Making encrypted loaders.
Henk MullerASD lecture, November 24, 2010 (^5)
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Languages - Tools
We use XC, C, C++, Java, Perl, Python, XC, Assembly code, (and Verilog)
C, C++, Java are used to build all customer facing tools
- For example, LLVM (the C compiler that we use) is largely written in C++
Perl and Python are used internally for scripting
- Munching data files
- Overnight build
- Overnight regression
- etc.
Henk MullerASD lecture, November 24, 2010 (^7)
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Languages - Applications
XC is used to implement most programs that run on our Hardware
- C with real time and concurrency built in
Assembly code
- Used whenever we cannot get there.
- When problems are challenging the limits of our hardware, and we need to get the extra 20%
Compilers need many man years of investment before they can generate mature code
- The XC compiler is good, but hand written code can still beat it.
Henk MullerASD lecture, November 24, 2010 (^8)
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XMOS software How does one program a device like this?
- Processor speed is ok (order of magnitude slower than a Pentium)
- Processor I/O and real time behaviour is superior to competition.
- But processor memory is limited to 64 K (single chip)
Current software memory requirements:
- Firefox requires 230 MBytes of memory; 4000 times XCore
- Current compilers requires 10-100 MBytes of memory.
- Linux needs tens of megabytes just to do nothing (better than Windows that requires a GByte to do nothing). Compare with
- Old fashioned Unix (ran in 32 KBytes)
- ZX-80 micro (1KByte ram)
- BBC micro (16 KByte ram)
Henk MullerASD lecture, November 24, 2010 (^10)
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Bloat ware
Current systems are build to bloat:
- In order to shorten development times
- Borrow libraries of which you only use a fraction of the code
- Rather than represent a set as a bit vector, use a ’HashSet()’
- These are good abstractions if you have the oomph.
- They are bad abstractions if you aim for low-power.
Lets elaborate on the HashSet - and show efficiency, limitations, and sharing.
Henk MullerASD lecture, November 24, 2010 (^11)
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Memory use per component
Design each component to manage its own memory
- Component needs a predictable and precise amount of memory
- Don’t add 1 just for the sake of it
Now make sure that all components fit together statically
- Do not guess that it may fit
- Do not rely on malloc
- Memory may run out dynamically (when the aeroplane lands)
- Malloc needs memory to register memory
- Malloc needs program code too. Everybody loses.
Henk MullerASD lecture, November 24, 2010 (^13)
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Data flow: pass data around
When component 1 is done with the data, pass it on to component 2.
- Do not just store it in a buffer where component 2 can pick it up later.
Buffers have a place in data flow management:
- Buffers can smooth data flow.
- Buffers do not increase bandwidth
- Buffers increase latency
- Buffers take a lot of memory
- Buffers make it difficult to analyse code
Henk MullerASD lecture, November 24, 2010 (^14)
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Conclusions
Memory is a scarce resource
- Think as hard about using memory.
Could give the same talk about power-aware software:
- Power is a scarce resource, think twice about doing unnecessary operations
Interestingly: Low memory designs are usually more efficient
- It takes a lot of power and time to fill memory...
Designing software is hard.
Henk MullerASD lecture, November 24, 2010 (^16)
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