Can GNOME startup time be improved via ld flags?
By alanc on Jul 11, 2005
Bryan Cantrill, Master DTrace Guru, First Class, spent some time today looking at what exactly GNOME is doing when you login to a Java Desktop System session on Solaris, and posted his findings to his weblog. (The current JDS on Solaris is based on GNOME 2.6, since that's what was the stable release last year when Solaris 10 hit feature freeze. The JDS team is working on an update to GNOME 2.10 now.)
One of the things Bryan found was that a large part of the I/O time was spent loading shared object text. I took a quick look at some of the binaries and libraries using elfdump, and noticed that there were no signs of using flags that could reduce the time needed to load shared libraries at process startup. Some of these (like -z lazyload) defer work until later - others (like -z combreloc) reduce the work needed whenever it happens.
I sent some suggestions to the JDS team on using these flags and others to improve this and suggested especially reading the Performance Considerations chapter of the Solaris Libraries and Linkers Guide for more ideas. I also cc'ed the linker gurus, and Senior Linker Alien Rod Evans added a suggestion to try out the check_rtime perl script on the binaries to check for the recommended flags and whether any of the libraries linked against aren't really needed. It's currently set up for use in the build system of the OS/Networking consolidation (the portion of the Solaris sources already released via OpenSolaris), but should be adaptable to the JDS build system or in fact, any project that wants to try to optimize it's library/linker use on Solaris.
Unfortunately, just tweaking the flags will mostly help Solaris, but the GNU binutils ld used on Linux and some other platforms offers some similar functionality - it recognizes many of the same -z options for instance, though I haven't tried them to see how they compare.
Something that may help more on both platforms is ensuring the libraries listed in the various .pc files for GNOME only list the direct requirements, not all the dependencies they depend on as well. For instance, look at what is linked into every program on Solaris that uses the gtk toolkit:
alanc@unknown:~  pkg-config --libs gtk+-2.0 -lgtk-x11-2.0 -lgdk-x11-2.0 -latk-1.0 -lgdk_pixbuf-2.0 -lm -lmlib -lpangoxft-1.0 -lpangox-1.0 -lpango-1.0 -lgobject-2.0 -lgmodule-2.0 -lglib-2.0But if you run elfdump -d /usr/lib/libgtk-x11-2.0.so you'll see libgtk-x11-2.0.so already lists those dependencies, so duplicating them in the applications simply wastes time as the linker at runtime will load libgtk-x11-2.0.so and have to check the same list of libraries it already checked in the application (though it should find it's already taken care of them and doesn't duplicate all the work). Additionally it hardcodes in the applications knowledge of the internals and backends used that they shouldn't need to know about, and makes it harder to change or replace one of them. While all those libraries need to be listed when statically linking, or on older systems (mainly pre-ELF I think), the pkg-config entries should be streamlined when using ELF shared libraries on modern systems.