Tag Archives: ERTS

When is the Book Coming Out?

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”So Erik, When is the book coming out?”

Last week I attended and spoke at Erlang Factory San Francisco.

The number one question that I got, and I got it from basically everyone I have ever met before, was ”When is the book coming out?”.

It was really nice to see that there was such an interest for the book.

The sad part is that the answer, at least in part, is: ”I don’t know”.

It is unfortunately late already. I was planning for a release in early 2014, and clearly that didn’t happen. I have only written about one third of the content. None of the chapters I have written has been edited, proofread, rewritten or even really finished. 

There is a huge amount of work left.

In the summer and fall of 2013 I again ended up as a manager at Klarna with the responsibility to get ”Kred” or ”Klarna Online”, our (well Klarna’s) OLTP system, to survive the Christmas load. The system was in a slightly unstable state at the beginning, but the Kred Core team managed to get it stable, responsive and really performing well straight through the Christmas rush. Unfortunately this took all of my time and energy, and the book writing got put on hold.

Fortunately, this led me to finally take the step and resign. 

The plan now is to take a week of vacation here in California, and then I will work full time on the book. If all goes well I should be able to finish most of the writing by early summer. Then perhaps I can get reviewer comments by the end of the summer and have a ”final” version ready by September or so.

From now on I will also try to keep you posted on the progress here on the blog.

I'm writing a book, I got the page numbers done.

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The Erlang Tagging Scheme

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Erlang is dynamically typed. That is, types will be checked at runtime and if a type error occurs an exception is thrown. The compiler does not check the types at compile time, unlike in a statically typed language like C or Java where you can get a type error during compilation.

These aspects of the Erlang type system, strongly statically typed with an order on the types puts some constraints on the implementation of the language. In order to be able to check and compare types at runtime each Erlang term has to carry its type with it. This is solved by tagging the terms.

In the memory representation of an Erlang term a few bits are reserved for a type tag. For performance reasons the terms are divided into immediate and boxed terms. An immediate term can fit into a machine word, that is, in a register or on a stack slot. A boxed term consists of two parts: a tagged pointer and a number of words stored on the process heap. The boxes stored on the heap have a header and a body, unless it is a list. Currently ERTS uses a staged tag scheme, the history and reasoning behind the this scheme is explained in a technical report from the HiPE group. (See http://www.it.uu.se/research/publications/reports/2000-029/)

The tagging scheme is implemented in erl_term.h. The basic idea is to use the least significant bits for tags. Since most modern CPU architectures aligns 32- and 64-bit words, there are at least two bits that are “unused” for pointers. These bits can be used as tags instead. Unfortunately those two bits are not enough for all the types in Erlang, more bits are therefore used as needed.

The current (R15-R16) tagging scheme looks like this:

 aaaaaaaaaaaaaaaaaaaaaaaaaatttt00 HEADER (see below)
 pppppppppppppppppppppppppppppp01 CONS
 pppppppppppppppppppppppppppppp10 BOXED (pointer to header)
 iiiiiiiiiiiiiiiiiiiiiiiiiiii0011 PID
 iiiiiiiiiiiiiiiiiiiiiiiiiiii0111 PORT
 iiiiiiiiiiiiiiiiiiiiiiiiii001011 ATOM
 iiiiiiiiiiiiiiiiiiiiiiiiii011011 CATCH
 iiiiiiiiiiiiiiiiiiiiiiiiii111011 NIL (i always zero...)
 iiiiiiiiiiiiiiiiiiiiiiiiiiii1111 SMALL_INT

 aaaaaaaaaaaaaaaaaaaaaaaaaa000000 ARITYVAL (Tuple)
 vvvvvvvvvvvvvvvvvvvvvvvvvv000100 BINARY_AGGREGATE       |
 vvvvvvvvvvvvvvvvvvvvvvvvvv001x00 BIGNUM with sign bit   |
 vvvvvvvvvvvvvvvvvvvvvvvvvv010000 REF                    |
 vvvvvvvvvvvvvvvvvvvvvvvvvv010100 FUN                    | THINGS
 vvvvvvvvvvvvvvvvvvvvvvvvvv011000 FLONUM                 |
 vvvvvvvvvvvvvvvvvvvvvvvvvv011100 EXPOR                  |
 vvvvvvvvvvvvvvvvvvvvvvvvvv100000 REFC_BINARY |          |
 vvvvvvvvvvvvvvvvvvvvvvvvvv100100 HEAP_BINARY | BINARIES |
 vvvvvvvvvvvvvvvvvvvvvvvvvv101000 SUB_BINARY  |          |
 vvvvvvvvvvvvvvvvvvvvvvvvvv101100 Not used
 vvvvvvvvvvvvvvvvvvvvvvvvvv110000 EXTERNAL_PID  |        |
 vvvvvvvvvvvvvvvvvvvvvvvvvv110100 EXTERNAL_PORT | EXT    |
 vvvvvvvvvvvvvvvvvvvvvvvvvv111000 EXTERNAL_REF  |        | 
 vvvvvvvvvvvvvvvvvvvvvvvvvv111100 Not used

 

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The Erlang Runtime System

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I recently started writing a book about the Erlang Runtime System. The plan is to have the book done by the end of 2013 and published early 2014. During this time I plan to do some blog posts about the topics I am writing on so stay tuned.

Currently I am thinking of writing the following chapters:

  1. Introduction
  2. The Compiler
  3. Processes and the PCB
  4. The Erlang Virtual Machine: BEAM
  5. Modules and The BEAM File Format
  6. Scheduling
  7. The Erlang Type System and Tags
  8. The Memory Subsystem: Stacks, Heaps and Garbage Collection
  9. Advanced data structures (ETS, DETS, Mnesia)
  10. Different Types of Calls, Linking and Hot Code Loading
  11. BEAM Instructions (25p)
  12. Platform Independence
  13. IO, Ports and Networking
  14. BIFs NIFs and Linked in Drivers
  15. Native code/li>
  16. Building ERTS
  17. The Shell
  18. Operation and maintenance
  19. Crash dumps
  20. The Debugger
  21. Tracing and Profiling
  22. Tweaking and optimizing

Comments on the suggested content are welcome.

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