Improving bulk performance in debhelper

Since debhelper/10.3, there has been a number of performance related changes.  The vast majority primarily improves bulk performance or only have visible effects at larger “input” sizes.

Most visible cases are:

• dh + dh_* now scales a lot better for large number of binary packages.  Even more so with parallel builds.
• Most dh_* tools are now a lot faster when creating many directories or installing files.
• dh_prep and dh_clean now bulk removals.
• dh_install can now bulk some installations.  For a concrete corner-case, libssl-doc went from approximately 11 seconds to less than a second.  This optimization is implicitly disabled with –exclude (among other).
• dh_installman now scales a lot better with many manpages.  Even more so with parallel builds.
• dh_installman has restored its performance under fakeroot (regression since 10.2.2)

For debhelper, this mostly involved:

• avoiding fork+exec of commands for things doable natively in perl.  Especially, when each fork+exec only process one file or dir.
• bulking as many files/dirs into the call as possible, where fork+exec is still used.
• caching / memorizing slow calls (e.g. in parts of pkgfile inside Dh_Lib)
• adding an internal API for dh to do bulk check for pkgfiles. This is useful for dh when checking if it should optimize out a helper.
• and, of course, doing things in parallel where trivially possible.

How to take advantage of these improvements in tools that use Dh_Lib:

• If you use install_{file,prog,lib,dir}, then it will come out of the box.  These functions are available in Debian/stable.  On a related note, if you use “doit” to call “install” (or “mkdir”), then please consider migrating to these functions instead.
• If you need to reset owner+mode (chown 0:0 FILE + chmod MODE FILE), consider using reset_perm_and_owner.  This is also available in Debian/stable.
  • CAVEAT: It is not recursive and YMMV if you do not need the chown call (due to fakeroot).
• If you have a lot of items to be processed by a external tool, consider using xargs().  Since 10.5.1, it is now possible to insert the items anywhere in the command rather than just in the end.
• If you need to remove files, consider using the new rm_files function.  It removes files and silently ignores if a file does not exist. It is also available since 10.5.1.
• If you need to create symlinks, please consider using make_symlink (available in Debian/stable) or make_symlink_raw_target (since 10.5.1).  The former creates policy compliant symlinks (e.g. fixup absolute symlinks that should have been relative).  The latter is closer to a “ln -s” call.
• If you need to rename a file, please consider using rename_path (since 10.5).  It behaves mostly like “mv -f” but requires dest to be a (non-existing) file.
• Have a look at whether on_pkgs_in_parallel() / on_items_in_parallel() would be suitable for enabling parallelization in your tool.
  • The emphasis for these functions is on making parallelization easy to add with minimal code changes.  It pre-distributes the items which can lead to unbalanced workloads, where some processes are idle while a few keeps working.

Credits:

I would like to thank the following for reporting performance issues, regressions or/and providing patches.  The list is in no particular order:

• Helmut Grohne
• Kurt Roeckx
• Gianfranco Costamagna
• Iain Lane
• Sven Joachim
• Adrian Bunk
• Michael Stapelberg

Should I have missed your contribution, please do not hesitate to let me know.