distcc - distributed C/C++/ObjC compiler

distcc <compiler> [COMPILER OPTIONS]

distcc [COMPILER OPTIONS]

<compiler> [COMPILER OPTIONS]

distcc distributes compilation of C code across several machines on a network. distcc should always generate the same results as a local compile, is simple to install and use, and is often much faster than a local compile.

distcc sends the complete preprocessed source code and compiler arguments across the network for each job, so the machines do not need to share a filesystem, have the same headers or libraries installed, or have synchronized clocks.

Compilation is driven by a "client" machine, which is typically the developer's workstation or laptop. The distcc client runs on this machine, as does make, the preprocessor, the linker, and other stages of the build process. Any number of "volunteer" machines help the client to build the program, by running the distccd(1) daemon, C compiler and assembler as required.

distcc can run across either TCP sockets (on port 3632 by default), or through a tunnel command such as ssh(1). For TCP connections the volunteers must run the distccd(1) daemon either directly or from inetd. For SSH connections distccd must be installed but should not be listening for connections.

TCP connections should only be used on secure networks because there is no user authentication or protection of source or object code. SSH connections are typically 25% slower because of processor overhead for encryption, although this can vary greatly depending on CPUs, network and the program being built.

distcc is intended to be used with GNU Make's -j option, which runs several compiler processes concurrently. distcc spreads the jobs across both local and remote CPUs. Because distcc is able to distribute most of the work across the network a higher concurrency level can be used than for local builds. The -j value should normally be set to about twice the total number of available CPUs, to allow for some tasks being blocked waiting for disk or network IO. distcc can also work with other build control tools such as SCons.

It is strongly recommended that you install the same compiler version on all machines participating in a build. Incompatible compilers may cause mysterious compile or link failures.

1

For each machine, download distcc, unpack, and install.

2

On each of the servers, run distccd --daemon optionally with --allow options to restrict access.

3

Put the names of the servers in your environment: $ export DISTCC_HOSTS='localhost red green blue'

4

Build! $ make -j8 CC=distcc

distcc only ever runs the compiler and assembler remotely. The preprocessor must always run locally because it needs to access various header files on the local machine which may not be present, or may not be the same, on the volunteer. The linker similarly needs to examine libraries and object files, and so must run locally.

The compiler and assembler take only a single input file (the preprocessed source) and produce a single output (the object file). distcc ships these two files across the network and can therefore run the compiler/assembler remotely.

Fortunately, for most programs running the preprocessor is relatively cheap, and the linker is called relatively infrequent, so most of the work can be distributed.

distcc examines its command line to determine which of these phases are being invoked, and whether the job can be distributed.

Most options passed to distcc are interpreted as compiler options. Two options are understood by distcc itself:

--help

Displays summary instructions.

--version

Displays the distcc client version.

There are three different ways to call distcc, to suit different circumstances:

distcc can be installed under the name of the real compiler, to intercept calls to it and run them remotely. This "masqueraded" compiler has the widest compatibility with existing source trees, and is convenient when you want to use distcc for all compilation. The fact that distcc is being used is transparent to the makefiles.

distcc can be prepended to compiler command lines, such as "distcc cc -c hello.c" or CC="distcc gcc". This is convenient when you want to use distcc for only some compilations or to try it out, but can cause trouble with some makefiles or versions of libtool that assume $CC does not contain a space.

Finally, distcc can be used directly as a compiler. "cc" is always used as the name of the real compiler in this "implicit" mode. This can be convenient for interactive use when "explicit" mode does not work but is not really recommended for new use.

Remember that you should not use two methods for calling distcc at the same time. If you are using a masquerade directory, don't change CC and/or CXX, just put the dirirectory early on your PATH. If you're not using a masquerade directory, you'll need to either change CC and/or CXX, or modify the makefile(s) to call distcc explicitly.

The basic idea is to create a "masquerade directory" which contains links from the name of the real compiler to the distcc binary. This directory is inserted early on the PATH, so that calls to the compiler are intercepted and distcc is run instead. distcc then removes itself from the PATH to find the real compiler.

For example:

# mkdir /usr/lib/distcc/bin
# cd /usr/lib/distcc/bin
# ln -s ../../../bin/distcc gcc
# ln -s ../../../bin/distcc cc
# ln -s ../../../bin/distcc g++
# ln -s ../../../bin/distcc c++

Then, to use distcc, a user just needs to put the directory /usr/lib/distcc/bin early in the PATH, and have set a host list in DISTCC_HOSTS or a file. distcc will handle the rest.

Note that this masquerade directory must occur on the PATH earlier than the directory that contains the actual compilers of the same names, and that any auxiliary programs that these compilers call (such as as or ld) must also be found on the PATH in a directory after the masquerade directory since distcc calls out to the real compiler with a PATH value that has all directory up to and including the masquerade directory trimmed off.

It is possible to get a "recursion error" in masquerade mode, which means that distcc is somehow finding itself again, not the real compiler. This can indicate that you have two masquerade directories on the PATH, possibly because of having two distcc installations in different locations. It can also indicate that you're trying to mix "masqueraded" and "explicit" operation.

ccache is a program that speeds software builds by caching the results of compilations. ccache is normally called before distcc, so that results are retrieved from a normal cache. Some experimentation may be required for idiosyncratic makefiles to make everything work together.

The most reliable method is to set

CCACHE_PREFIX=" distcc"

This tells ccache to run distcc as a wrapper around the real compiler. ccache still uses the real compiler to detect compiler upgrades.

ccache can then be run using either a masquerade directory or by setting

CC=" ccache gcc"

As of version 2.2, ccache does not cache compilation from preprocessed source and so will never get a cache hit if it is run from distccd or distcc. It must be run only on the client side and before distcc to be any use.

A "host list" tells distcc which machines to use for compilation. In order, distcc looks in the $DISTCC_HOSTS environment variable, the user's $DISTCC_DIR/hosts file, and the system-wide host file. If no host list can be found, distcc emits a warning and compiles locally.

The host list is a simple whitespace separated list of host specifications. The simplest and most common form is a host names, such as

localhost red green blue

distcc prefers hosts towards the start of the list, so machines should be listed in descending order of speed. In particular, when only a single compilation can be run (such as from a configure script), the first machine listed is used.

Placing localhost at the right point in the list is important to getting good performance. Because overhead for running jobs locally is low, localhost should normally be first. However, it is important that the client have enough cycles free to run the local jobs and the distcc client. If the client is slower than the volunteers, or if there are many volunteers, then the client should be put later in the list or not at all. As a general rule, if the aggregate CPU speed of the client is less than one fifth of the total, then the client should be left out of the list.

Performance depends on the details of the source and makefiles used for the project, and the machine and network speeds. Experimenting with different settings for the host list and -j factor may improve performance.

The syntax is

  DISTCC_HOSTS = HOSTSPEC ...
  HOSTSPEC = LOCAL_HOST | SSH_HOST | TCP_HOST | OLDSTYLE_TCP_HOST
  LOCAL_HOST = localhost[/LIMIT]
  SSH_HOST = [USER]@HOSTID[/LIMIT][:COMMAND][OPTIONS]
  TCP_HOST = HOSTID[:PORT][/LIMIT][OPTIONS]
  OLDSTYLE_TCP_HOST = HOSTID[/LIMIT][:PORT][OPTIONS]
  HOSTID = HOSTNAME | IPV4
  OPTIONS = ,OPTION[OPTIONS]
  OPTION = lzo

Here are some individual examples of the syntax:

localhost

The literal word "localhost" is interpreted specially to cause compilations to be directly executed, rather than passed to a daemon on the local machine. If you do want to connect to a daemon on the local machine for testing, then give the machine's IP address or real hostname. (This will be slower.)

IPV4

A literal IPv4 address, such as 10.0.0.1

HOSTNAME

A hostname to be looked up using the resolver.

:PORT

Connect to a specified decimal port number, rather than the default of 3632.

@HOSTID

Connect to the host over SSH, rather than TCP. Options for the SSH connection can be set in ~/.ssh/config

USER@

Connect to the host over SSH as a specified username.

:COMMAND

Connect over SSH, and use a specified path to find the distccd server. This is normally only needed if for some reason you can't install distccd into a directory on the default PATH for SSH connections. Use this if you get errors like "distccd: command not found" in SSH mode.

/LIMIT

A decimal limit can be added to any host specification to restrict the number of jobs that this client will send to the machine. The limit defaults to four per host (two for localhost), but may be further restricted by the server. You should only need to increase this for servers with more than two processors.

,lzo

Enables LZO compression for this TCP or SSH host.

Here is an example demonstrating some possibilities:

localhost/2 @bigman/16:/opt/bin/distccd oldmachine:4200/1
# cartman is down
distant/3,lzo

Comments are allowed in host specifications. Comments start with a hash/pound sign (#) and run to the end of the line.

If a host in the list is not reachable distcc will emit a warning and ignore that host for about one minute.

The lzo host option specifies that LZO compression should be used for data transfer, including preprocessed source, object code and error messages. Compression is usually economical on networks slower than 100Mbps, but results may vary depending on the network, processors and source tree.

Enabling compression makes the distcc client and server use more CPU time, but less network traffic. The compression ratio is typically 4:1 for source and 2:1 for object code.

Using compression requires both client and server to use at least release 2.9 of distcc. No server configuration is required: the server always responds with compressed replies to compressed requests.

If the compiler name is an absolute path, it is passed verbatim to the server and the compiler is run from that directory. For example:

distcc /usr/local/bin/gcc-3.1415 -c hello.c

If the compiler name is not absolute, or not fully qualified, distccd's PATH is searched. When distcc is run from a masquerade directory, only the base name of the compiler is used. The client's PATH is used only to run the preprocessor and has no effect on the server's path.

Both the distcc client and server impose timeouts on transfer of data across the network. This is intended to detect hosts which are down or unreachable, and to prevent compiles hanging indefinitely if a server is disconnected while in use. If a client-side timeout expires, the job will be re-run locally.

The timeouts are not configurable at present.

Error messages or warnings from local or remote compilers are passed through to diagnostic output on the client.

distcc can supply extensive debugging information when the verbose option is used. This is controlled by the DISTCC_VERBOSE environment variable on the client, and the --verbose option on the server. For troubleshooting, examine both the client and server error messages.

The exit code of distcc is normally that of the compiler: zero for successful compilation and non-zero otherwise.

distcc distinguishes between "genuine" errors such as a syntax error in the source, and "accidental" errors such as a networking problem connecting to a volunteer. In the case of accidental errors, distcc will retry the compilation locally unless the DISTCC_FALLBACK option has been disabled.

If the compiler exits with a signal, distcc returns an exit code of 128 plus the signal number.

distcc internal errors cause an exit code between 100 and 127. In particular

100

General distcc failure.

105

Out of memory.

110

Compiler not found.

111

Recursive call to distcc.

116

No hosts defined and fallbacks disabled.

(Others are listed in exitcode.h.)

If $DISTCC_HOSTS is not set, distcc reads a host list from either $DISTCC_DIR/hosts or a system-wide configuration file set at compile time. The file locations are shown in the output from distcc --help

distcc creates a number of temporary and lock files underneath the temporary directory.

distcc's behaviour is controlled by a number of environment variables. For most cases nothing need be set if the host list is stored in a file.

DISTCC_HOSTS

Space-separated list of volunteer host specifications.

DISTCC_VERBOSE

If set to 1, distcc produces explanatory messages on the standard error stream or in the log file. This can be helpful in debugging problems. Bug reports should include verbose output.

DISTCC_LOG

Log file to receive messages from distcc itself, rather than stderr.

DISTCC_FALLBACK

By default distcc will compile locally if it fails to distribute a job to the intended machine, or if no host list can be found. If this variable is set to 0 then fallbacks are disabled and those compilations will simply fail. Note that this does not affect jobs which must always be local such as linking.

DISTCC_SAVE_TEMPS

If set to 1, temporary files are not deleted after use. Good for debugging, or if your disks are too empty.

DISTCC_TCP_CORK

If set to 0, disable use of "TCP corks", even if they're present on this system. Using corks normally helps pack requests into fewer packets and aids performance. This should normally be left enabled.

DISTCC_SSH

Specifies the command used for opening SSH connections. Defaults to "ssh" but may be set to a different connection command such as "lsh" or "tsocks-ssh" that accepts a similar command line. The command is not split into words and is not executed through the shell.

DISTCC_DIR

Per-user configuration directory to store lock files and state files. By default ~/.distcc/ is used.

TMPDIR

Directory for temporary files such as preprocessor output. By default /tmp/ is used.

UNCACHED_ERR_FD

If set and if DISTCC_LOG is not set, distcc errors are written to the file descriptor identified by this variable. This variable is intended mainly for automatic use by ccache, which sets it to avoid caching transient errors such as network problems.

Cross compilation means building programs to run on a machine with a different processor, architecture, or operating system to where they were compiled. distcc supports cross compilation, including teams of mixed-architecture machines, although some changes to the compilation commands may be required.

The compilation command passed to distcc must be one that will execute properly on every volunteer machine to produce an object file of the appropriate type. If the machines have different processors, then simply using distcc cc will probably not work, because that will normally invoke the volunteer's native compiler.

Machines with the same CPU but different operating systems may not necessarily generate compatible .o files.

Several different gcc configurations can be installed side-by-side on any machine. If you build gcc from source, you should use the --program-suffix configuration options to cause it to be installed with a name that encodes the gcc version and the target platform.

The recommended convention for the gcc name is TARGET-gcc-VERSION such as i686-linux-gcc-3.2 . GCC 3.3 will install itself under this name, in addition to TARGET-gcc and, if it's native, gcc-VERSION and gcc .

The compiler must be installed under the same name on the client and on every volunteer machine.

If you think you have found a distcc bug, please see the file reporting-bugs.txt in the documentation directory for information on how to report it.

Some makefiles have missing or extra dependencies that cause incorrect or slow parallel builds. Recursive make is inefficient and can leave processors unnecessarily idle for long periods. (See Recursive Make Considered Harmful by Peter Miller.) Makefile bugs are the most common cause of trees failing to build under distcc. Alternatives to Make such as SCons can give much faster builds for some projects.

Using different versions of gcc can cause confusing build problems because the header files and binary interfaces have changed over time, and some distributors have included incompatible patches without changing the version number. distcc does not protect against using incompatible versions. Compiler errors about link problems or declarations in system header files are usually due to mismatched or incorrectly installed compilers.

Due to limitations in gcc, gdb may not be able to automatically find the source files for programs built using distcc in some circumstances. The gdb directory command can be used. This should be fixed in gcc 3.4.

gcc's -MD option can produce output in the wrong directory if the source and object files are in different directories and the -MF option is not used. There is no perfect solution because of incompatible changes between gcc versions. Explicitly specifying the dependency output file with -MF will fix the problem.

TCP mode connections should only be used on trusted networks.

Including slow machines in the list of volunteer hosts can slow the build down.

When distcc or ccache is used on NFS, the filesystem must be exported with the no_subtree_check option to allow reliable renames between directories.

The compiler can be invoked with a command line gcc hello.c to both compile and link. distcc doesn't split this into separate parts, but rather runs the whole thing locally.

Other known bugs may be documented on http://distcc.samba.org/

distcc was written by Martin Pool <mbp@sourcefrog.net>, with the co-operation of many scholars including Wayne Davison, Frerich Raabe, Dimitri Papadopoulos and others noted in the NEWS file. Please report bugs to <distcc@lists.samba.org>.

You are free to use distcc. distcc (including this manual) may be copied, modified or distributed only under the terms of the GNU General Public Licence version 2 or later. distcc comes with absolutely no warrany. A copy of the GPL is included in the file COPYING.

distccd(1), ccache(1), gcc(1), make(1) http://distcc.samba.org/ http://ccache.samba.org/