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diff --git a/doc/design/assistant/blog/day_6__polish.mdwn b/doc/design/assistant/blog/day_6__polish.mdwn new file mode 100644 index 000000000..dd1239626 --- /dev/null +++ b/doc/design/assistant/blog/day_6__polish.mdwn @@ -0,0 +1,50 @@ +Since my last blog, I've been polishing the `git annex watch` command. + +First, I fixed the double commits problem. There's still some extra +committing going on in the `git-annex` branch that I don't understand. It +seems like a shutdown event is somehow being triggered whenever +a git command is run by the commit thread. + +I also made `git annex watch` run as a proper daemon, with locking to +prevent multiple copies running, and a pid file, and everything. +I made `git annex watch --stop` stop it. + +--- + +Then I managed to greatly increase its startup speed. At startup, it +generates "add" events for every symlink in the tree. This is necessary +because it doesn't really know if a symlink is already added, or was +manually added before it starter, or indeed was added while it started up. +Problem was that these events were causing a lot of work staging the +symlinks -- most of which were already correctly staged. + +You'd think it could just check if the same symlink was in the index. +But it can't, because the index is in a constant state of flux. The +symlinks might have just been deleted and re-added, or changed, and +the index still have the old value. + +Instead, I got creative. :) We can't trust what the index says about the +symlink, but if the index happens to contian a symlink that looks right, +we can trust that the SHA1 of its blob is the right SHA1, and reuse it +when re-staging the symlink. Wham! Massive speedup! + +--- + +Then I started running `git annex watch` on my own real git annex repos, +and noticed some problems.. Like it turns normal files already checked into +git into symlinks. And it leaks memory scanning a big tree. Oops.. + +--- + +I put together a quick screencast demoing `git annex watch`. + +<video controls src="http://joeyh.name/screencasts/git-annex-watch.ogg"></video> + +While making the screencast, I noticed that `git-annex watch` was spinning +in strace, which is bad news for powertop and battery usage. This seems to +be a [GHC bug](http://bugs.debian.org/677096) also affecting Xmonad. I +tried switching to GHC's threaded runtime, which solves that problem, but +causes git-annex to hang under heavy load. Tried to debug that for quite a +while, but didn't get far. Will need to investigate this further.. +Am seeing indications that this problem only affects ghc 7.4.1; in +particular 7.4.2 does not seem to have the problem. |