USE nptl nptlonly и threads. Что использовать ?

Заметил, что включены nptl и threads USE-флаги. Как я понимаю nptl - это современная замена POSIX-threads ? Что использовать, а что выключить ?

Portage 2.1.7.15 (default/linux/x86/10.0, gcc-4.4.2, glibc-2.11-r1, 2.6.32-gentoo i686)
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System uname: 
Timestamp of tree: Fri, 18 Dec 2009 22:00:01 +0000                                                 
ccache version 2.4 [enabled]                                                                       
app-shells/bash:     4.0_p35                                                                       
dev-java/java-config: 2.1.10                                                                       
dev-lang/python:     2.6.4, 3.1.1-r1                                                               
dev-util/ccache:     2.4-r8                                                                        
dev-util/cmake:      2.8.0                                                                         
sys-apps/baselayout: 2.0.1                                                                         
sys-apps/openrc:     0.5.3                                                                         
sys-apps/sandbox:    2.2                                                                           
sys-devel/autoconf:  2.13, 2.64                                                                    
sys-devel/automake:  1.9.6-r2, 1.10.3, 1.11.1                                                      
sys-devel/binutils:  2.20                                                                          
sys-devel/gcc-config: 1.4.1                                                                        
sys-devel/libtool:   2.2.6b                                                                        
virtual/os-headers:  2.6.30-r1                                                                     
ACCEPT_KEYWORDS="x86 ~x86"                                                                         
ACCEPT_LICENSE="* -@EULA"                                                                          
CBUILD="i686-pc-linux-gnu"                                                                         
CFLAGS="-O2 -march=native -pipe -fomit-frame-pointer -floop-interchange -floop-block -floop-strip-mine"
CHOST="i686-pc-linux-gnu"                                                                              
CONFIG_PROTECT="/etc /usr/share/X11/xkb /usr/share/config /var/lib/hsqldb"                             
CONFIG_PROTECT_MASK="/etc/ca-certificates.conf /etc/env.d /etc/env.d/java/ /etc/fonts/fonts.conf /etc/gconf /etc/gentoo-release /etc/revdep-rebuild /etc/sandbox.d /etc/terminfo"                                                                             
CXXFLAGS="-O2 -march=native -pipe -fomit-frame-pointer -floop-interchange -floop-block -floop-strip-mine"                      
DISTDIR="/usr/portage/distfiles"                                                                                               
FEATURES="assume-digests ccache distlocks fixpackages news nodoc noinfo parallel-fetch protect-owned sandbox sfperms strict unmerge-logs unmerge-orphans userfetch"                                                                                           
GENTOO_MIRRORS="http://mirror.yandex.ru/gentoo-distfiles/ ftp://mirror.yandex.ru/gentoo-distfiles/ http://distfiles.gentoo.org/distfiles/"                                                                                                                    
LANG="ru_RU.UTF-8"                                                                                                             
LC_ALL="ru_RU.UTF-8"
LDFLAGS="-Wl,-O1"
LINGUAS="ru"
MAKEOPTS="-j2"
PKGDIR="/usr/portage/packages"
PORTAGE_CONFIGROOT="/"
PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --compress --force --whole-file --delete --stats --timeout=180 --exclude=/distfiles --exclude=/local --exclude=/packages"
PORTAGE_TMPDIR="/var/tmp"
PORTDIR="/usr/portage"
SYNC="rsync://secretstar/gentoo-portage"
USE="7zip X Xaw3d a52 aac acl acpi alsa amrnb amrwb berkdb bluetooth bzip2 cdr chm cli cracklib crypt cups cupsddk cxx dbus djvu dri dts dvd dvdr encode exif ffmpeg flac fortran gd gdbm gif gpm hal iconv ipv6 java jpeg kde lm_sensors lua mad matroska mikmod mmx modules mp2 mp3 mpeg mudflap musepack ncurses nls nptl nptlonly nvidia ogg opengl openmp oscar pam pcre pdf perl plasma png pppd pulseaudio python qt4 quicktime rar readline reflection scanner sdl session slang smp spell spl sqlite sse sse2 sse3 ssl ssse3 svg sysfs tcpd theora threads tiff truetype twolame unicode usb userlocales vdpau vorbis win32codecs wxwidgets x264 x86 xcomposite xorg xvid zip zlib" ALSA_CARDS="hda-intel" ALSA_PCM_PLUGINS="adpcm alaw asym copy dmix dshare dsnoop empty extplug file hooks iec958 ioplug ladspa lfloat linear meter mmap_emul mulaw multi null plug rate route share shm softvol" APACHE2_MODULES="actions alias auth_basic authn_alias authn_anon authn_dbm authn_default authn_file authz_dbm authz_default authz_groupfile authz_host authz_owner authz_user autoindex cache dav dav_fs dav_lock deflate dir disk_cache env expires ext_filter file_cache filter headers include info log_config logio mem_cache mime mime_magic negotiation rewrite setenvif speling status unique_id userdir usertrack vhost_alias" ELIBC="glibc" INPUT_DEVICES="evdev" KERNEL="linux" LCD_DEVICES="bayrad cfontz cfontz633 glk hd44780 lb216 lcdm001 mtxorb ncurses text" LINGUAS="ru" RUBY_TARGETS="ruby18" SANE_BACKENDS="hp" USERLAND="GNU" VIDEO_CARDS="nvidia"
Unset:  CPPFLAGS, CTARGET, EMERGE_DEFAULT_OPTS, FFLAGS, INSTALL_MASK, PORTAGE_COMPRESS, PORTAGE_COMPRESS_FLAGS, PORTAGE_RSYNC_EXTRA_OPTS, PORTDIR_OVERLAY