pena portage # emerge -uDa1 --jobs=4 world

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild     U  ] app-emulation/virtualbox-modules-4.1.2 [4.0.12]
[ebuild  N     ] dev-ruby/net-ssh-2.2.0  USE="-doc -test" RUBY_TARGETS="ruby18 -ree18 -ruby19" 
[ebuild     U  ] dev-ruby/bunny-0.7.4 [0.6.0-r1]
[ebuild     U  ] dev-ruby/yajl-ruby-0.8.3 [0.8.2]
[ebuild     U  ] dev-ruby/systemu-2.3.0 [2.2.0]
[ebuild     U  ] dev-ruby/mime-types-1.16-r3 [1.16-r2]
[ebuild  N     ] dev-ruby/polyglot-0.3.2  USE="-test" RUBY_TARGETS="ruby18 -jruby -ree18 -ruby19" 
[ebuild     U  ] dev-ruby/bundler-1.0.18 [1.0.15]
[ebuild  N     ] dev-ruby/treetop-1.4.10  USE="-doc -test" RUBY_TARGETS="ruby18 -ree18" 
[ebuild     U  ] dev-ruby/json-1.5.2 [1.4.6] RUBY_TARGETS="-ruby19%" 
[ebuild     U  ] dev-ruby/rest-client-1.6.3 [1.6.1] RUBY_TARGETS="-ree18%" 
[ebuild  N     ] dev-ruby/net-ssh-gateway-1.1.0  USE="-doc -test" RUBY_TARGETS="ruby18 -ree18" 
[ebuild  N     ] dev-ruby/net-ssh-multi-1.1  USE="-doc -test" RUBY_TARGETS="ruby18 -ree18" 
[ebuild     U  ] app-admin/chef-0.10.2 [0.9.12]
[ebuild     U  ] x11-base/xorg-server-1.10.4 [1.10.3]
[ebuild     U  ] mail-client/thunderbird-6.0 [5.0] USE="-custom-cflags%" LINGUAS="-be% -bn% -bn_BD% -br% -el% -gd% -gl% -id% -rm% -ta% -ta_LK%" 
[ebuild     U  ] www-client/chromium-14.0.835.94 [13.0.782.109]
[ebuild     U  ] sys-devel/clang-2.9-r1 [2.9]
[ebuild     U  ] www-client/firefox-6.0 [5.0-r2] USE="crashreporter%*" 
[ebuild     U  ] app-admin/sudo-1.8.2 [1.8.1_p2]
[ebuild     U  ] net-libs/libssh-0.5.1 [0.5.0]
[ebuild     U  ] dev-python/PyQt4-4.8.5 [4.8.4]
[ebuild     U  ] app-text/xournal-0.4.5-r2 [0.4.5-r1]
[ebuild     U  ] app-emulation/virtualbox-bin-4.1.2 [4.0.12]
[ebuild     U  ] app-text/texlive-core-2011-r4 [2011-r3]
[ebuild     U  ] kde-base/kdelibs-4.7.0-r1 [4.7.0] USE="-upnp%" 
[ebuild     U  ] www-plugins/adobe-flash-11.0.1.60_beta201108082 [11.0.1.60_beta201107131-r1]
[ebuild     U  ] kde-base/kwin-4.7.0-r1 [4.7.0]
[ebuild     U  ] kde-base/kdeplasma-addons-4.7.0-r1 [4.7.0]
[ebuild     UD ] net-fs/samba-3.5.11 [3.6.0_rc3-r1] USE="-smbtav2%" 
[uninstall     ] net-fs/cifs-utils-5.0 
[blocks b      ] <net-fs/samba-3.6_rc1 ("<net-fs/samba-3.6_rc1" is blocking net-fs/cifs-utils-5.0)
[blocks b      ] net-fs/cifs-utils ("net-fs/cifs-utils" is blocking net-fs/samba-3.5.11)

Would you like to merge these packages? [Yes/No] 
>>> Verifying ebuild manifests
>>> Running pre-merge checks for x11-base/xorg-server-1.10.4
>>> Running pre-merge checks for kde-base/kdelibs-4.7.0-r1
>>> Starting parallel fetch
>>> Emerging (1 of 30) app-emulation/virtualbox-modules-4.1.2
>>> Emerging (2 of 30) dev-ruby/net-ssh-2.2.0
>>> Emerging (3 of 30) dev-ruby/bunny-0.7.4
>>> Emerging (4 of 30) dev-ruby/yajl-ruby-0.8.3
>>> Failed to emerge app-emulation/virtualbox-modules-4.1.2, Log file:
>>>  '/var/tmp/portage/app-emulation/virtualbox-modules-4.1.2/temp/build.log'
>>> Failed to emerge dev-ruby/net-ssh-2.2.0, Log file:
>>>  '/var/tmp/portage/dev-ruby/net-ssh-2.2.0/temp/build.log'
>>> Failed to emerge dev-ruby/bunny-0.7.4, Log file:
>>>  '/var/tmp/portage/dev-ruby/bunny-0.7.4/temp/build.log'
>>> Failed to emerge dev-ruby/yajl-ruby-0.8.3, Log file:
>>>  '/var/tmp/portage/dev-ruby/yajl-ruby-0.8.3/temp/build.log'
>>> Jobs: 0 of 30 complete, 4 failed                Load avg: 0.25, 2.23, 2.51
*** Resuming merge...
Calculating dependencies... done!

 * Messages for package app-emulation/virtualbox-modules-4.1.2:

 *   CONFIG_IOMMU_SUPPORT:       is not set when it should be.
 *   CONFIG_DMAR:        is not set when it should be.
 *   CONFIG_PCI_STUB:    is not set when it should be.
 * Please check to make sure these options are set correctly.
 * Failure to do so may cause unexpected problems.
 * These sources have not yet been prepared.
 * We cannot build against an unprepared tree.
 * To resolve this, please type the following:
 * 
 * # cd /usr/src/linux
 * # make oldconfig
 * # make modules_prepare
 * 
 * Then please try merging this module again.
 * ERROR: app-emulation/virtualbox-modules-4.1.2 failed (setup phase):
 *   Kernel sources need compiling first
 * 
 * Call stack:
 *                         ebuild.sh, line  56:  Called pkg_setup
 *   virtualbox-modules-4.1.2.ebuild, line  33:  Called linux-mod_pkg_setup
 *                  linux-mod.eclass, line 587:  Called check_kernel_built
 *                 linux-info.eclass, line 644:  Called die
 * The specific snippet of code:
 *              die "Kernel sources need compiling first"
 * 
 * If you need support, post the output of 'emerge --info =app-emulation/virtualbox-modules-4.1.2',
 * the complete build log and the output of 'emerge -pqv =app-emulation/virtualbox-modules-4.1.2'.
 * The complete build log is located at '/var/tmp/portage/app-emulation/virtualbox-modules-4.1.2/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/app-emulation/virtualbox-modules-4.1.2/temp/die.env'.
 * S: '/var/tmp/portage/app-emulation/virtualbox-modules-4.1.2/work'

 * Messages for package dev-ruby/net-ssh-2.2.0:

 * ERROR: dev-ruby/net-ssh-2.2.0 failed (install phase):
 *   Unable to generate gemspec file.
 * 
 * Call stack:
 *     ebuild.sh, line   56:  Called src_install
 *   environment, line 2857:  Called ruby-ng_src_install
 *   environment, line 2525:  Called _ruby_each_implementation 'each_ruby_install'
 *   environment, line  428:  Called _ruby_invoke_environment 'ruby18' 'each_ruby_install'
 *   environment, line  473:  Called each_ruby_install
 *   environment, line  890:  Called each_fakegem_install
 *   environment, line  875:  Called ruby_fakegem_install_gemspec
 *   environment, line 2682:  Called die
 * The specific snippet of code:
 *       fi ) || die "Unable to generate gemspec file.";
 * 
 * If you need support, post the output of 'emerge --info =dev-ruby/net-ssh-2.2.0',
 * the complete build log and the output of 'emerge -pqv =dev-ruby/net-ssh-2.2.0'.
 * The complete build log is located at '/var/tmp/portage/dev-ruby/net-ssh-2.2.0/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-ruby/net-ssh-2.2.0/temp/environment'.
 * S: '/var/tmp/portage/dev-ruby/net-ssh-2.2.0/work/ruby18/net-ssh-2.2.0'

 * Messages for package dev-ruby/bunny-0.7.4:

 * ERROR: dev-ruby/bunny-0.7.4 failed (install phase):
 *   Unable to generate gemspec file.
 * 
 * Call stack:
 *     ebuild.sh, line   56:  Called src_install
 *   environment, line 2782:  Called ruby-ng_src_install
 *   environment, line 2450:  Called _ruby_each_implementation 'each_ruby_install'
 *   environment, line  355:  Called _ruby_invoke_environment 'ruby18' 'each_ruby_install'
 *   environment, line  400:  Called each_ruby_install
 *   environment, line  819:  Called each_fakegem_install
 *   environment, line  804:  Called ruby_fakegem_install_gemspec
 *   environment, line 2607:  Called die
 * The specific snippet of code:
 *       fi ) || die "Unable to generate gemspec file.";
 * 
 * If you need support, post the output of 'emerge --info =dev-ruby/bunny-0.7.4',
 * the complete build log and the output of 'emerge -pqv =dev-ruby/bunny-0.7.4'.
 * The complete build log is located at '/var/tmp/portage/dev-ruby/bunny-0.7.4/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-ruby/bunny-0.7.4/temp/environment'.
 * S: '/var/tmp/portage/dev-ruby/bunny-0.7.4/work/ruby18/bunny-0.7.4'

 * Messages for package dev-ruby/yajl-ruby-0.8.3:

 * ERROR: dev-ruby/yajl-ruby-0.8.3 failed (install phase):
 *   Unable to generate gemspec file.
 * 
 * Call stack:
 *     ebuild.sh, line   56:  Called src_install
 *   environment, line 2828:  Called ruby-ng_src_install
 *   environment, line 2496:  Called _ruby_each_implementation 'each_ruby_install'
 *   environment, line  390:  Called _ruby_invoke_environment 'ruby18' 'each_ruby_install'
 *   environment, line  435:  Called each_ruby_install
 *   environment, line  861:  Called each_fakegem_install
 *   environment, line  837:  Called ruby_fakegem_install_gemspec
 *   environment, line 2653:  Called die
 * The specific snippet of code:
 *       fi ) || die "Unable to generate gemspec file.";
 * 
 * If you need support, post the output of 'emerge --info =dev-ruby/yajl-ruby-0.8.3',
 * the complete build log and the output of 'emerge -pqv =dev-ruby/yajl-ruby-0.8.3'.
 * The complete build log is located at '/var/tmp/portage/dev-ruby/yajl-ruby-0.8.3/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-ruby/yajl-ruby-0.8.3/temp/environment'.
 * S: '/var/tmp/portage/dev-ruby/yajl-ruby-0.8.3/work/ruby18/yajl-ruby-0.8.3'
[ebuild     U  ] dev-ruby/mime-types-1.16-r3 [1.16-r2]
[ebuild  N     ] dev-ruby/polyglot-0.3.2  USE="-test" RUBY_TARGETS="ruby18 -jruby -ree18 -ruby19" 
[ebuild     U  ] dev-ruby/systemu-2.3.0 [2.2.0]
[ebuild     U  ] dev-ruby/bundler-1.0.18 [1.0.15]
[ebuild  N     ] dev-ruby/treetop-1.4.10  USE="-doc -test" RUBY_TARGETS="ruby18 -ree18" 
[ebuild     U  ] dev-ruby/json-1.5.2 [1.4.6] RUBY_TARGETS="-ruby19%" 
[ebuild     U  ] dev-ruby/rest-client-1.6.3 [1.6.1] RUBY_TARGETS="-ree18%" 
[ebuild     U  ] x11-base/xorg-server-1.10.4 [1.10.3]
[ebuild     U  ] mail-client/thunderbird-6.0 [5.0] USE="-custom-cflags%" LINGUAS="-be% -bn% -bn_BD% -br% -el% -gd% -gl% -id% -rm% -ta% -ta_LK%" 
[ebuild     U  ] www-client/chromium-14.0.835.94 [13.0.782.109]
[ebuild     U  ] sys-devel/clang-2.9-r1 [2.9]
[ebuild     U  ] www-client/firefox-6.0 [5.0-r2] USE="crashreporter%*" 
[ebuild     U  ] app-admin/sudo-1.8.2 [1.8.1_p2]
[ebuild     U  ] net-libs/libssh-0.5.1 [0.5.0]
[ebuild     U  ] dev-python/PyQt4-4.8.5 [4.8.4]
[ebuild     U  ] app-text/xournal-0.4.5-r2 [0.4.5-r1]
[ebuild     U  ] app-text/texlive-core-2011-r4 [2011-r3]
[ebuild     U  ] kde-base/kdelibs-4.7.0-r1 [4.7.0] USE="-upnp%" 
[ebuild     U  ] www-plugins/adobe-flash-11.0.1.60_beta201108082 [11.0.1.60_beta201107131-r1]
[ebuild     U  ] kde-base/kwin-4.7.0-r1 [4.7.0]
[ebuild     U  ] kde-base/kdeplasma-addons-4.7.0-r1 [4.7.0]
[ebuild     UD ] net-fs/samba-3.5.11 [3.6.0_rc3-r1] USE="-smbtav2%" 
[uninstall     ] net-fs/cifs-utils-5.0 
[blocks b      ] <net-fs/samba-3.6_rc1 ("<net-fs/samba-3.6_rc1" is blocking net-fs/cifs-utils-5.0)
[blocks b      ] net-fs/cifs-utils ("net-fs/cifs-utils" is blocking net-fs/samba-3.5.11)

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

dev-ruby/json:0

  (dev-ruby/json-1.5.2::gentoo, ebuild scheduled for merge) pulled in by
    (no parents that aren't satisfied by other packages in this slot)

  (dev-ruby/json-1.4.6::gentoo, installed) pulled in by
    <dev-ruby/json-1.4.7[ruby_targets_ruby18] required by (app-admin/chef-0.9.12::gentoo, installed)


It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously.  If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously. You may want to try a larger value of
the --backtrack option, such as --backtrack=30, in order to see if
that will solve this conflict automatically.

For more information, see MASKED PACKAGES section in the emerge man
page or refer to the Gentoo Handbook.

 * 
 * The following 4 packages have failed to build or install:
 * 
 *  (app-emulation/virtualbox-modules-4.1.2::gentoo, ebuild scheduled for merge), Log file:
 *   '/var/tmp/portage/app-emulation/virtualbox-modules-4.1.2/temp/build.log'
 *  (dev-ruby/net-ssh-2.2.0::gentoo, ebuild scheduled for merge), Log file:
 *   '/var/tmp/portage/dev-ruby/net-ssh-2.2.0/temp/build.log'
 *  (dev-ruby/bunny-0.7.4::gentoo, ebuild scheduled for merge), Log file:
 *   '/var/tmp/portage/dev-ruby/bunny-0.7.4/temp/build.log'
 *  (dev-ruby/yajl-ruby-0.8.3::gentoo, ebuild scheduled for merge), Log file:
 *   '/var/tmp/portage/dev-ruby/yajl-ruby-0.8.3/temp/build.log'
 *