Configuring compile options
To optimize Gentoo, it is possible to set a couple of variables which impacts the behavior of Portage, Gentoo's officially supported package manager. All those variables can be set as environment variables (using export) but that isn't permanent. To keep the settings, Portage reads in the /etc/portage/make.conf file, a configuration file for Portage.
A commented listing of all possible variables can be found in /mnt/gentoo/usr/share/portage/config/make.conf.example. For a successful Gentoo installation only the variables that are mentioned below need to be set.
Fire up an editor (in this guide we use nano) to alter the optimization variables we will discuss hereafter.
nano -w /mnt/gentoo/etc/portage/make.conf
From the make.conf.example file it is obvious how the file should be structured: commented lines start with "#", other lines define variables using the VARIABLE="content" syntax. Several of those variables are discussed next.
CFLAGS and CXXFLAGS
The CFLAGS and CXXFLAGS variables define the optimization flags for GCC C and C++ compilers respectively. Although those are defined generally here, for maximum performance one would need to optimize these flags for each program separately. The reason for this is because every program is different. However, this is not manageable, hence the definition of these flags in the make.conf file.
In make.conf one should define the optimization flags that will make the system the most responsive generally. Don't place experimental settings in this variable; too much optimization can make programs behave bad (crash, or even worse, malfunction).
We will not explain all possible optimization options. To understand them all, read the GNU Online Manual(s), the gcc info page (info gcc - only works on a working Linux system) or the gcc man-page (man gcc). The make.conf.example file itself also contains lots of examples and information; don't forget to read it too.
A first setting is the
-mtune= flag, which specifies the name of the target architecture. Possible options are described in the make.conf.example file (as comments). A commonly used value is native as that tells the compiler to select the target architecture of the current system (the one users are installing Gentoo on).
A second one is the
-O flag (that is a capital O, not a zero), which specifies the gcc optimization class flag. Possible classes are s (for size-optimized), 0 (zero - for no optimizations), 1, 2 or even 3 for more speed-optimization flags (every class has the same flags as the one before, plus some extras).
-O2 is the recommended default.
-O3 is known to cause problems when used system-wide, so we recommend to stick to
Another popular optimization flag is
-pipe (use pipes rather than temporary files for communication between the various stages of compilation). It has no impact on the generated code, but uses more memory. On systems with low memory, gcc might get killed. In that case, do not use this flag.
-fomit-frame-pointer (which doesn't keep the frame pointer in a register for functions that don't need one) might have serious repercussions on the debugging of applications.
When the CFLAGS and CXXFLAGS variables are defined, combine the several optimization flags in one string. The default values contained in the stage3 archive that is unpacked should be good enough. The following one is just an example:
# Compiler flags to set for all languages
# Use the same settings for both variables
Although the GCC optimization article has more information on how the various compilation options can affect a system, the Safe CFLAGS article may be a more practical place for beginners to start optimizing their systems.
The MAKEOPTS variable defines how many parallel compilations should occur when installing a package. A good choice is the number of CPUs (or CPU cores) in the system plus one, but this guideline isn't always perfect.
Using a large number of jobs can significantly impact memory consumption. A good recommendation is to have at least 2 GiB of RAM for every job specified (so, e.g.
-j6 requires at least 12 GiB). To avoid running out of memory, lower the number of jobs to fit the available memory.
When using parallel emerges (
--jobs), the effective number of jobs run can grow exponentially (up to make jobs multiplied by emerge jobs). This can be worked around by running a localhost-only distcc configuration that will limit the number of compiler instances per host.
TODO: set ACCEPT_KEYWORDS to unstable on bleeding edge arches
Ready, set, go!
Update the /mnt/gentoo/etc/portage/make.conf file to match personal preference and save (nano users would hit Ctrl+x).