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update the README
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Andrew Tridgell
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README
140
README
@ -9,147 +9,11 @@ I wrote ccache because I wanted to get a bit more speed out of a
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compiler cache and I wanted to remove some of the limitations of the
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shell-script version.
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Installation
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------------
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Please see the manual page and documentation at
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http://ccache.samba.org/
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There are two ways to use ccache. You can either prefix your compile
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commands with "ccache" or you can create a symbolic link between
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ccache and the names of your compilers. The first method is most
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convenient if you just want to try out ccache or wish to use it for
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some specific projects. The second method is most useful for when you
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wish to use ccache for all your compiles.
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To install for usage by the first method just copy ccache to somewhere
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in your path.
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To install for the second method do something like this:
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cp ccache /usr/local/bin/
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ln -s /usr/local/bin/ccache /usr/local/bin/gcc
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ln -s /usr/local/bin/ccache /usr/local/bin/cc
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This will work as long as /usr/local/bin comes before the path to gcc
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(which is usually in /usr/bin). After installing you may wish to run
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"which gcc" to make sure that the correct link is being used.
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Setting cache limits
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--------------------
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Run "ccache -h" to see a list of options. The main ones you may wish
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to look at are "ccache -M" and "ccache -F" for setting the cache size
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limits.
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You can use "ccache -s" to look at the cache hit/miss statistics.
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Configuration
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-------------
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Configuration of ccache is done via a number of environment variables
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and via ccache commands. In most cases you won't need any of these as
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the defaults will be fine.
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CCACHE_DIR
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the CCACHE_DIR environment variable specifies where ccache will
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keep its cached compiler output. The default is "$HOME/.ccache".
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CCACHE_LOGFILE
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If you set the CCACHE_LOGFILE environment variable then ccache will
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write some log information on cache hits and misses in that
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file. This is useful for tracking down problems.
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CCACHE_PATH
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You can optionally set CCACHE_PATH to a colon separated path where ccache will
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look for the real compilers. If you don't do this then ccache will
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look for the first executable in the normal PATH that isn't a link
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to ccache itself.
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CCACHE_DISABLE
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If you set the environment variable CCACHE_DISABLE then ccache will
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just call the real compiler, bypassing the cache completely.
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Differences
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-----------
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The biggest differences between Erik's compilercache script and ccache
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are:
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- ccache is written in C, which makes it a bit faster (calling out to
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external programs is mostly what slowed down the scripts).
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- ccache can automatically find the real compiler
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- ccache keeps statistics on hits/misses
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- ccache can do automatic cache management
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- ccache can cache compiler output that includes warnings. In many
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cases this gives ccache a much higher cache hit rate.
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- ccache can handle argument lists with spaces
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- compilercache does much smarter token unification using a C
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lexer. This means that reformatting of source code is much more
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likely to result in a cache miss with ccache than with
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compilercache. I may add this in future, although it is perhaps
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debatable whether the much slower parsing is worth it.
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Note however that ccache is a very new piece of code (as of March
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2002) whereas Erik's scripts have had a *lot* more testing.
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The overall speedup compared to compilercache is quite dramatic. For
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compiling rsync I get:
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normal build 20.74 seconds
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compilercache uncached 28.9 seconds
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compilercache cached 10.5 seconds
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ccache uncached 24.6 seconds
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ccache cached 4.6 seconds
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Cleaning size management
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------------------------
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By default ccache has no limit on the cache size. You can set a limit
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using the "ccache -M" and "ccache -F" options, which set the size and
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number of files limits.
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When these limits are reached ccache will reduce the cache to 20%
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below the numbers you specified in order to avoid doing the cache
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clean operation too often.
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How it works
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------------
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The basic idea is to detect when you are compiling exactly the same
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code a 2nd time and use the previously compiled output. You detect
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that it is the same code by forming a hash of:
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- the pre-processor output from running the compiler with -E
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- the command line options
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- the real compilers size and modification time
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- any stderr output generated by the compiler
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These are hashed using md4 (a strong hash) and a cache file is formed
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based on that hash result. When the same compilation is done a second
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time ccache is able to supply the correct compiler output (including
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all warnings etc) from the cache.
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ccache has been carefully written to always produce exactly the same
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compiler output that you would get without the cache. If you ever
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discover a case where ccache changes the output of your compiler then
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please let me know.
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===================
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Andrew Tridgell
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http://samba.org/~tridge/
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bugs@ccache.samba.org
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March 2002
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