If you don't want to have the majority of builds run in the background while you are busy on the system, but you don't want to automatically install software in the background when you are not behind your desk, then perhaps you can settle for using binary packages. I'm not saying you need to setup a build server and such or do your updates first in a chroot.

No, what this tip is for is to use the --buildpkgonly parameter of emerge at night, building some of your software (often the larger ones) as binary packages only (storing those in the ${PKGDIR} which defaults to /usr/portage/packages). When you are then on your system, you can run the update with binary package included:

# emerge -puDk world

To use --buildpkgonly, all package(s) that Portage wants to build must have all their dependencies met. If not, then the build will not go through and you're left with no binary packages at all. So what we do is to create a script that looks at the set of packages that would be build, and then one for one building the binary package.

When ran, the script will attempt to build binary packages for those that have no dependency requirements anymore. Those builds will then create a binary package but will not be merged on the system. When you later update your system, including binary packages, those packages that have been build during the night will be merged quickly, reducing the build load on your system while you are working on it.

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#!/bin/sh

LIST=$(mktemp);

emerge -puDN --color=n --columns --quiet=y world | awk '{print $2}' > ${LIST};

for PACKAGE in $(cat ${LIST});
do
  printf "Building binary package for ${PACKAGE}... "
  emerge -uN --quiet-build --quiet=y --buildpkgonly ${PACKAGE};
  if [[ $? -eq 0 ]];
  then
    echo "ok";
  else
    echo "failed";
  fi
done

I ran this a couple of days ago when -uDN world showed 46 package updates (including a few hefty ones like chromium). After running this script, 35 of them had a binary package ready so the -uDN world now only needed to build 11 packages, merging the remainder from binary packages.