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@@ -0,0 +1,55 @@ + + distcc-pump -- a free distributed C/C++ compiler system + + originally by Martin Pool <mbp@samba.org> + + extended at Google Inc. + + by Fergus Henderson, Nils Klarlund, Manos Renieris, and Craig Silverstein + + +distcc-pump improves on distcc by distributing not only compilation but also +preprocessing to distcc servers. For an introduction to distcc, see +distcc/README. + +The distcc-pump contributes an include server process that exists during the +build. The include server parses and analyzes source and header files. It runs +on the workstation that initiates the build. The include server analyzes each +header file only a few times, sometimes just once. In contrast, ordinary distcc +invokes the preprocessor hundreds of times on average for each header file of a +large build. + +With the distcc-pump, a static analysis algorithm inspects includes and computes +their possible values according to an overapproximation semantics. The resulting +dependency graph among header files is stored in the include server, which then +acts as a cache for include analysis. + +The include server compresses source files into a temporary directory as they +are encountered. In this way, a given source file is compressed only once during +the build. + +It may happen that a header file is included via an absolutely specified include +directory such as -I/absolute/path. But on the compilation server the path +-I/absolute/path does not exist; instead the server places foo.h under +/server_temporary_path/absolute/path for some /server_temporary_path root +directory. This directory has no meaning on the workstation. Before compressing +foo.h, the include server therefore inserts a #line directive in foo.h in such +cases to inform the preprocessor that the real location is /absolute/path. + +The distcc-pump client asks the include server for the list of compressed files +that constitute the transitive closure of the source file to be compiled. It +then spools these files to a distcc-pump server. The distcc-pump server unpacks +these files in the /server_temporary_path directory before preprocessing and +compiling. The server also rewrites include options, such as -I's, to reflect +the new locations of the files on the server. The .d and the .o files are both +returned to the distcc-pump client. + +The distcc-pump is able to distribute compilations up to 10X faster than distcc. +But because building also involves linking and perhaps generation of source +files the overall speed-up of the build time is variable. + +The distcc-pump was developed to be used with large clusters of distcc servers, +providing hundreds of CPUs. With versions of gcc >= 4.1.1, the distcc-pump will +probably not show performance gains using clusters of say only ten CPUs. The +preprocessor running on the workstation is fast enough to keep that many +machines busy. |