Nicolas Pitre <nico@xxxxxxx> wrote: > On Thu, 3 Apr 2008, Jakub Narebski wrote: > > > One of bigger hindrances, as I understand it, in developing pack v4 > > was the fact that it didn't offer that much of improvement in typical > > cases for the work needed... but perhaps "your" repository would be > > good showcase for pack v4. > > The biggest hindrance for pack v4 is actually the lack of a native > runtime tree walking, and having both tree object formats properly and > optimally abstracted has not been looked at yet. > > Speed is the primary goal for pack v4. The fact that it also provides a > 10% pack reduction is only consequential. But without native tree > walking we must recreate the legacy tree format on the fly each time a > tree object is loaded which dwarfs any improvements pack v4 is aiming > for (yes it is still a little bit faster than pack v3 nevertheless, but > not yet significantly enough to overcome the incompatibility costs). Even though we don't have native tree walking, I think the right way to do this is to put in pack v4 with "canonical tree, canonical commit" mode, where it inflates its native tree/commit encoding into the canonical forms, then come back later with native walking. Canonical mode is still faster than pack v2 inflate is for these types, so it does (slightly) boost rev-list performance. It might chop a solid 30% off the CPU time jgit spends in its equivilant of revision.c, and that's without teaching jgit to use the native pack v4 encoding directly. Once we have it in we can experiment with the necessary abstractions to handle the two different available encodings, and allowing higher level code to switch back and forth between them as objects come from loose or pack v2, and from pack v4. One of the things we wanted to do was boost path limiter performance by matching on tree name ids when walking a pack v4 native tree, but fall back to the string based memcmp when walking a canonical tree. That won't be easy to design without the two different encodings being available at the lower level in sha1_file.c. Just my rapidly declining .02 bush peso. > Nicolas (who wishes he was still a student with plenty of hacking time) Don't we all. :-) -- Shawn. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
