LIBSOLV-HISTORY(3) LIBSOLV LIBSOLV-HISTORY(3)NAMElibsolv-history - how the libsolv library came into existence
HISTORY
This project was started in May 2007 when the zypp folks decided to
switch to a database to speed up installation. As I am not a big fan of
databases, I (mls) wondered if there would be really some merit of
using one for solving, as package dependencies of all packages have to
be read in anyway.
Back in 2002, I researched that using a dictionary approach for storing
dependencies can reduce the packages file to 1/3 of its size. Extending
this idea a bit more, I decided to store all strings and relations as
unique 32-bit numbers. This has three big advantages:
· because of the unification, testing whether two strings are equal
is the same as testing the equality of two numbers, thus very fast
· much space is saved, as numbers do not take up as much space as
strings the internal memory representation does not take more space
on a 64-bit system where a pointer is twice the size of a 32-bit
number
Thus, the solv format was created, which stores a repository as a
string dictionary, a relation dictionary and then all packages
dependencies. Tests showed that reading and merging multiple solv
repositories takes just some milliseconds.
Early solver experiments
Having a new repository format was one big step, but the other area
where libzypp needed improvement was the solver. Libzypp’s solver was a
port from the Red Carpet solver, which was written to update packages
in an already installed system. Using it for the complete installation
progress brought it to its limits. Also, the added extensions like
support for weak dependencies and patches made it fragile and
unpredictable.
As I was not very pleased with the way the solver worked, I looked at
other solver algorithms. I checked smart, yum and apt, but could not
find a convincing algorithm. My own experiments also were not very
convincing, they worked fine for some problems but failed miserably for
other corner cases.
Using SAT for solving
SUSE’s hack week at the end of June 2007 turned out to be a turning
point for the solver. Googling for solver algorithms, I stumbled over
some note saying that some people are trying to use SAT algorithms to
improve solving on Debian. Looking at the SAT entry in Wikipedia, it
was easy to see that this indeed was the missing piece: SAT algorithms
are well researched and there are quite some open source
implementations. I decided to look at the minisat code, as it is one of
the fastest solvers while consisting of too many lines of code.
Of course, directly using minisat would not work, as a package solver
does not need to find just one correct solution, but it also has to
optimize some metrics, i.e. keep as many packages installed as
possible. Thus, I needed to write my own solver incorporation the ideas
and algorithms used in minisat. This wasn’t very hard, and at the end
of the hack week the solver calculated the first right solutions.
Selling it to libzypp
With those encouraging results, I went to Klaus Kaempf, the system
management architect at SUSE. We spoke about how to convince the team
to make libzypp switch to the new solver. Fortunately, libzypp comes
with a plethora of solver test cases, so we decided to make the solver
pass most of the test cases first. Klaus wrote a "deptestomatic"
implementation to check the test cases. Together with Stephan Kulow,
who is responsible for the openSUSE distribution, we tweaked and
extended the solver until most of the test cases looked good.
Duncan Mac-Vicar Prett, the team lead of the YaST team, also joined
development by creating Ruby bindings for the solver. Later, Klaus
improved the bindings and ported them to some other languages.
The attribute store
The progress with the repository format and the solver attracted
another hacker to the project: Michael Matz from the compiler team. He
started with improving the repository parsers so that patches and
content files also generate solvables. After that, he concentrated on
storing all of the other metadata of the repositories that are not used
for solving, like the package summaries and descriptions. At the end of
October, a first version of this "attribute store" was checked in. Its
design goals were:
· space efficient storage of attributes
· paging/on demand loading of data
· page compression
The first version of the attribute store used a different format for
storing information, we later merged this format with the solv file
format.
libzypp integration
Integration of the sat-solver into libzypp also started in October 2007
by Stefan Schubert and Michael Andres from the YaST team. The first
versions supported both the old solver and the new one by using the old
repository read functions and converting the old package data in-memory
into a sat solver pool. Solvers could be switched with the environment
variable ZYPP_SAT_SOLVER. The final decision to move to the new solver
was made in January of 2008, first just by making the new solver the
default one, later by completely throwing out the old solver code. This
had the advantage that the internal solvable storage could also be done
by using the solver pool, something Michael Matz already played with in
a proof of concept implementation showing some drastic speed gains. The
last traces of the old database code were removed in February.
AUTHOR
Michael Schroeder <mls@suse.de>
libsolv 09/20/2013 LIBSOLV-HISTORY(3)
