The slab list is a very memory-efficient logarithmic-time data structure.
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What Is a Slab List?

Slab lists are a memory efficient data structure, that can optionally keep data sorted in logarithmic time.

What Is libslablist?

libslablist is a C library that that implements slab lists. The concept of the Slab List is not tied to any particular language, just as the concept of an AVL Tree is can be implemented in any Turing-complete langauge.

Why Slab Lists?

I designed slab lists with quality and performance in mind. I had a lot of code using ad-hoc linked lists and pre-canned AVL Trees at the same time. While these are good solutions to well-known problems, I was tired of (a) using two different structures for the same data and (b) not being able to hold all of my data in memory -- I had to search it by chunks. So slab lists address both of those problems. They can be used for sorted and unsorted data, which allows us to use the same umem caches for both. They are also 4x to 5x more memory efficient than AVL Trees, allowing one to reduce the number of times one goes to disk.

Where Can I Use libslablist?

You can use this library from any C program that runs on Linux or Illumos. Support for FreeBSD is forthcoming. If you want a rock-solid data data structure that can implement a sorted set or even a simple sequential list, libslablist is more than up to the task. A lot of useful things can be implemented with sorted sets.

Where Is libslablist Used?

libslablist is reported to be used by developers in multiple stealth projects, to great effect. Publicly, however, libslablist is used in another project called libgraph. See:

libgraph is a library that manipulates graphs (in the graph-theory sense). libgraph uses libslablist as the backend for implementing a graph as a list of edges. Typically, a graph is implemented using a matrix or adjacency list to represent the edges. Using libslablist has the advantage of separating the data (the nodes) from the container (the edges connecting the nodes). Which means the same nodes can be present in multiple graphs, for example.

Where Can I Learn More?

Check out the development blog for historical or technical information:

See the block comments peppered throughout src/slablist_impl.h for an implementation overview. Trust me, you won't understand what's going on if you don't read those comments.

How Is The Code Organized?

The actual C implementation of the slab list is in the src/ directory.

All of the code in that directory follows the Illumos kernel coding style, sometimes referred to as BJNF (Bill Joy Normal Form).

Here is an explanation of what each file does.

  • slablist_impl.h: Common structures, constants, as well as detailed documentation of the implementation.

  • slablist.h: The consumer-facing function declarations and constants.

  • slablist_add.c: The element insertion routines.

  • slablist_rem.c: The element removal routines.

  • slablist_umem.c: The memory allocation routines.

  • slablist_find.c: The search routines. Everything for searching slabs, subslabs, and slablists.

  • slablist_cons.c: Slablist creation, destruction, reaping routines. Also, linking routines for slabs, subslabs, and small_lists. Also, sublayer attach/detach routines. Routines for converting between singly-linked-lists and slab lists. Finally foldr, foldl, and map routines, as well as their ranged variants. Basically, a dumping ground for everything that doesn't fit in _add, _rem, or _umem source files.

  • slablist_test.c: A large collection of testing routines. These routines sanity check the state of the slablist. For example, it checks that the number of elements in a slab never exceeds the maximum, and that a removal doesn't leave a gap in the slab, and so forth. Basically, these tests are like very powerful and very expensive ASSERTs. They are very expensive to run all the time, so they are wrapped in DTrace IS_ENABLED() probes. This means they won't be run unless we enable specific DTrace probes at run time. This means that they are always present but remain dormant until activated by someone. For this reason there is no DEBUG conditional-macro in the code. We ship with all of our probes and test functions, because they have negligible costs when disabled.

  • slablist_provider.d: A DTrace source file that defines the DTrace probes provided by the library.

  • slablist_provider.h: A header file generated from the slablist_provider.d file. It has to live in the tree, to facilitate compilation on systems where the dtrace command may not be present (such as Linux).

The tools/ directory contains tools that can be used in development, or that are used by the benchmarking code. The tests/ directory contains D scripts that trigger the test-code (see section below for more details). The bench/ directory contains code that generates R code that can be used to process data generated by the DTrace durint the running of the benchmarks. There is also a script that fires off R jobs, to generate plots in parallel (serial is too slow -- took 45 minutes to generate all plots, now takes 5). The build/ directory contains subdirectories that correspond to different operating systems, as well as a Makefile.master file which contains common variables that are used by makefiles within the OS-subdirectories. The build/$OS directory contains a makefile that can build the library and -- if DTrace is supported -- run benchmarks and generate plots.

How Stable Is The API?

The libslablist API is not yet stable. It is volatile and may change.

How Can I Install libslablist?

To install:

cd build/illumos
make install

How Does libslablist Perform Compared to $X?

Here are some comparisons, so that you can get an idea of how Slab Lists perform. First we will compare to uuavl. uuavl is the AVL Tree implementation used in the Illumos kernel. It is among the most memory-efficient and cpu-efficient implementations in the world.

uuavl uses 10% less time than libslablist on sequential input.

uuavl uses 365% more memory than libslablist on sequential input.

uuavl uses 7% less time than libslablist on random input.

uuavl uses 271% more memory than libslablist on random input.

You'll find a full report here:

Other AVL Tree implementations, such as those found in GNU libavl use 50% to 100% more memory than uuavl. Which makes them half as competitive as uuavl. GNU libavl implementations also perform as well as libslablist.

We have a bunch of foreign data structure implementations that we use to evaluate Slab List performance. Most of these are self-contained and can be built from this tree. Others are simply too large to be included, or were designed as shared objects. These include: libuutil, libredblack, and myskl. You'll have to fetch them and install them manually.

How Do I Run Those DTrace Tests?

The tests have to be activated using code written in DTrace's D language through the dtrace utility. We have some examples in the tests/ directory and they can be executed like so:

dtrace -c 'build/drv_gen ...' -L <libpath> -s tests/<script-name>.d

By default, libslablist uses the following library path for DTrace libraries: /opt/libslablist/include/dtrace.