Hi, and welcome to ccache!
Date: Fri, 25 Sep 2009 10:22:14 +0200
From: tom.dalton at eds.com
To: ccache at lists.samba.org
Subject: [ccache] Build Safety
Hi,
I think I know the answer to this question, but I want a second opinion!
In the past, I have used the Clearcase version control system, which has
it's own make system, clearmake. Clearmake provides some build caching
functionality (which it terms wink-ins), that allows it to 'wink-in'
build objects from other users views. I have experienced a lot of
problems with this system, because clearcase wouldn't detect that two
users were building the same file in a different environment (e.g.
external libraries at different versions. This caused me all sorts of
issues.
That's why it is not very used.
Is it possible for inconsistent environments between builds to cause
ccache to incorrectly use a cached object instead of recompiling? For
example, different builds of gcc, different external libraries etc? What
circumstances could cause this?
for each compile ccache makes a key with the following information:1. the
pre-processor output from running the compiler with -E
2. the command line options
3. the real compilers size and modification time
4. any stderr output generated by the compilerif something changes then it
recompiles and recache the new key/.o
so if you use a different external librairies, #1 and maybe #2 can change.
Secondly, if there is not much that can break ccache, is it possible to
use a shared cache (e.g. on an nfs-shared disk) to allow many users on
many hosts to cache build results across several versions of a single
product? (which will mean different gcc versions, libraries, even
kernels.)
if each users use different gcc versions, libraries, even kernels, then
sharing the cache is not useful. It's better to use it locally. Try to compile
your product from scratch witch ccache and recompile (clean before) and
compare the time. (ie: time make all). Here is my current stats at this moment:
cache directory /home/vince/projects/raaf/ccache
cache hit 16462
cache miss 11307
called for link 70
compile failed63
preprocessor error 4
not a C/C++ file1198
files in cache 8779
cache size 793.5 Mbytes
max cache size 976.6 Mbytes
like you see, I skip compilation of 16462 files locally.
Thanks for your help - ccache sounds really useful, but I want to make
sure there's no 'traps'!
Thanks,
Tom
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