From: Chris Lee Date: Thu, 04 Jan 2007 17:56:52 GMT Subject: Re: git-svnimport failed and now git-repack hates me Message-ID: <204011cb0701040956p11ea2cepe3efaaf396056ac0@mail.gmail.com> In-Reply-To: <204011cb0701031816hda8af9bw4d4a469c2b111339@mail.gmail.com> Accidentally sent this to just Linus instead of the list... On 1/3/07, Linus Torvalds wrote: > > So I'm using git 1.4.1, and I have been experimenting with importing > > the KDE sources from Subversion using git-svnimport. > > As one single _huge_ import? All the sub-projects together? I have to say, > that sounds pretty horrid. Unfortunately, that's how the KDE repo is organized. (I tried arguing against this when they were going to do the original import, but I lost the argument.) And git-svnimport doesn't appear to have any sort of method for splitting a gigantic svn repo into several smaller git repos. > > First issue I ran into: On a machine with 4GB of RAM, when I tried to > > do a full import, git-svnimport died after 309906 revisions, saying > > that it couldn't fork. > > > > Checking `top` and `ps` revealed that there were no git-svnimport > > processes doing anything, but all of my 4G of RAM was still marked as > > used by the kernel. I had to do sysctl -w vm.drop_caches=3 to get it > > to free all the RAM that the svn import had used up. > > I think that was just all cached, and all ok. The reason you didn't see > any git-svnimport was that it had died off already, and all your memory > was just caches. You could just have left it alone, and the kernel would > have started re-using the memory for other things even without any > "drop_caches". > > But what you did there didn't make anything worse, it was just likely had > no real impact. I got the tip about drop_caches from davej. Normally, when a process taking up a huge amount of memory exits, it shows a bunch of free memory in `top` and friends. I was a little bit surprised when that didn't happen this time. > However, it does sound like git-svnimport probably acts like git-cvsimport > used to, and just keeps too much in memory - so it's never going to act > really nicely.. > > It also looks like git-svnimport never repacks the repo, which is > absolutely horrible for performance on all levels. The CVS importer > repacks every one thousand commits or something like that. Yeah. I haven't bothered hacking git-svnimport yet - but it looks like having it automatically repack every thousand revisions or so would probably be a pretty big win. > > Now, after that, I tried doing `git-repack -a` because I wanted to see > > how small the packed archive would be (before trying to continue > > importing the rest of the revisions. There are at least another 100k > > revisions that I should be able to import, eventually.) > > I suspect you'd have been better off just re-starting, and using something > like > > while : > do > git svnimport -l 1000 <...> > .. figure out some way to decide if it's all done .. > git repack -d > done > > which would make svnimport act a bit more sanely, and repack > incrementally. That should make both the import much faster, _and_ avoid > any insane big repack at the end (well, you'd still want to do a "git > repack -a -d" at the end to turn the many smaller packs into a bigger one, > but it would be nicer). > > However, I don't know what the proper magic is for svnimport to do that > sane "do it in chunks and tell when you're all done". Or even better - to > just make it repack properly and not keep everything in memory. You can pass limits to svnimport to give it a revision to start at and another one to end at, so that wouldn't be too bad - I was thinking about working around it like that (so that i don't have to go poking around in the Perl code behind the svn importer). By default, if I had, say, one pack with the first 1000 revisions, and I imported another 1000, running 'git-repack' on its own would leave the first pack alone and create a new pack with just the second 1000 revisions, right? > > The repack finished after about nine hours, but when I try to do a > > git-verify-pack on it, it dies with this error message: > > > > error: Packfile > > .git/objects/pack/pack-540263fe66ab9398cc796f000d52531a5c6f3df3.pack > > SHA1 mismatch with itself > > That sounds suspiciously like the bug we had in out POWER sha1 > implementation that would generate the wrong SHA1 for any pack-file that > was over 512MB in size, due to an overflow in 32 bits (SHA1 does some > counting in _bits_, so 512MB is 4G _bits_), > > Now, I assume you're not on POWER (and we fixed that bug anyway - and I > think long before 1.4.1 too), but I could easily imagine the same bug in > some other SHA1 implementation (or perhaps _another_ overflow at the 1GB > or 2GB mark..). I assume that the pack-file you had was something horrid.. > > I hope this is with a 64-bit kernel and a 64-bit user space? That should > limit _some_ of the issues. But I would still not be surprised if your > SHA1 libraries had some 32-bit ("unsigned int") or 31-bit ("int") limits > in them somewhere - very few people do SHA1's over huge areas, and even > when you do SHA1 on something like a DVD image (which is easily over any > 4GB limit), that tends to be done as many smaller calls to the SHA1 > library routines. This is on a dual-CPU dual-core Opteron, running the AMD64 variant of Ubuntu's Edgy release (64-bit kernel, 64-bit native userland). The pack-file was around 2.3GB.