DMUCS – a Distributed Multi-User
(for use with distcc)
DMUCS is a system that allows a group of users to share a compilation
farm. Each compilation request from
each user will be sent to the fastest available machine, every time. The system has these fine qualities:
multiple users compiling simultaneously, and scales well to handle the new
multiple operating systems in the compilation farm.
- Uses all
processors of a multi-processor compilation host.
best use of compilation hosts with widely differing CPU speeds.
that a compilation host will not be overloaded by compilations.
into account the load on a host caused by non-compilation tasks.
the dynamic addition and removal of hosts to the compilation farm.
with distcc, which need not be altered in any way.
DMUCS consists of these (main) programs:
- dmucs: the
“host-server”. This application
reads a configuration file indicating the number of CPUs and the “power”
of each potential host in the compilation farm. It then receives over the network:
average information from each compilation host.
requests from compile tasks that need remote hosts on which to run.
requests from monitoring applications.
requests from an administrator.
dmucs maintains the database of hosts in the
compilation farm, and assigns hosts to compilation tasks, giving out the best
host/cpu available when the compilation task asks.
- gethost: a
compilation task uses gethost get a host/cpu from the dmucs
server. In general, a makefile
will perform a compilation this way:
gethost distcc gcc
gethost contacts the server to get a
host, which it puts into the environment variable DISTCC_HOSTS. gethost then calls the program given
to it. After that program ends, gethost
releases the assigned host back to the dmucs server.
- loadavg: the
administrator of the compilation farm must start this application on each
compilation host. loadavg
sends the load average of the compilation host to the dmucs server
periodically. The dmucs
server will “downgrade” a compilation host if the host’s load averages
goes too high.
- monitor: the
administrator (or anyone) may use this program to monitor the busy-ness of
the compilation farm. It displays
which hosts/cpus are available in the compilation farm, which hosts/cpus
have compilation tasks assigned to them, which hosts have been made
administratively unavailable, and which hosts are “silent” – i.e., the dmucs
server has not received a load average message from the compilation host
for a while.
- Get the latest version of DMUCS (0.6)
- dmucs installs using the standard ‘configure’, ‘make’, and ‘make
- Run configure. You may
wish to use the -–prefix argument to ‘configure’ if you don’t want the
executables, documentation, and configuration files to be installed under
/usr/local. Specifically, the
-–prefix argument will tell dmucs where to go to look for the
hosts-info file (/usr/local/share/dmucs/hosts-info, by default).
- Run make
<machine-name> is where you have chosen to run the dmucs
server. NOTE: this machine does not have
to be a powerful machine – I’ve used a very wimpy Sun Ultra 5 very
- Make sure you have your compilers in
place on each host. They must all
be found in the same directory on every compilation host. Also, make sure you have in place the
distcc executables (distcc and distccd, at least).
- Make sure the loadavg executable
can be accessed and executed on each host.
- Make sure the dmucs executable
can be accessed and executed on the machine you have chosen as your server
- Create a hosts-info file in the location
‘/usr/local/share/dmucs/hosts-info’ (or, if you specified –-prefix when
‘configure’ing, in <prefix>/share/dmucs/hosts-info). Here is a
sample hosts-info file:
# Format: machine number-of-cpus power-index
linux-comp-1 4 10
solaris-comp-1 2 5
solaris-comp-2 2 5
old-linux-comp-1 1 4
old-solaris-comp-3 1 2
18.104.22.168 1 2
As you can see, the format is simple: each line is
a host's name, then the number of cpus on that machine, and then the
"power index" of that machine. If you know that some machines
are much more powerful than others, then give them higher power indices.
The value must be >= 1. If you don't want to bother with a hosts-info file,
the system will still work, but all machines will be treated equally, and the
code will assume each machine has only 1 cpu.
(More information on how to choose power indices is given below.)
- Go to your host server machine and
install and start up the dmucs executable.
- Go to each compilation host
machine and run loadavg and start up the distccd daemons.
The output from the dmucs executable should show each host being
registered. (Alternatively, you
can use the shell script enable-host found in the dmucs/scripts
directory. This will start up the loadavg
and distccd programs on the host machine.)
- In your Makefiles (or Construct or
SConstruct files), make each compilation line look like this:
gethost distcc gcc ...
- Run your
build script and see the compilations being farmed out to the fastest
available compilation machines in your network.
Tips and Tricks
- If you
are having problems getting DMUCS to work, first make sure that you can
compile using distcc, but not dmucs. If this works, then you are most of the
way there. If not, then you should
go to the distcc web page for lots
of hints on getting distcc to work.
- If you
don’t know what values to give for the “power index” values in the
hosts-info file, you can do this:
- Set up
distcc to run a single compilation on a single compilation host.
- Repeat this
for the same file on each host, measuring the time it takes to compile
your slowest machine the power index 1.
Then, if the second slowest machine is twice as fast give it the
index 2. If you have a machine
that is 10 times faster, give it the index 10.
that it is often very useful to have multiple machines have the same
power index. In this case, dmucs
will randomly assign compilation tasks to the cpus for hosts with the
same power index.
Details on How DMUCS Selects Hosts
- dmucs stores
information about each compilation host in a tier that equals the
host’s power index. Thus, if there
are 3 machines all with power index 6, dmucs will put all machines
at tier 6.
- When dmucs
gets a host request, it randomly selects one of the cpus from one of the
hosts in the highest tier.
- When dmucs
gets a load average message from a compilation host indicating that
the load average on the machine is nearing 1.0 (or, more precisely, number-of-cpus
* 1.0), dmucs moves the cpus for that machine from tier power-index
to tier power-index – 1.
Thus, if a machine at tier 6 is getting overloaded, it is moved to
tier 5. Subsequent gethost
requests will result in allocations from remaining hosts at tier 6, thus
lightening the load on the machine that was “downgraded”.
- If a
machine’s load average stays above its power-index * 1.0 for more than 5
minutes, dmucs will move the machine to the “overloaded” state in
its internal database. No more
compilations will be assigned to the host until its load average comes
back down, and dmucs moves the machine back to its original tier.