Added a comment

doc/bugs/runs_of_of_memory_adding_2_million_files/comment_3_3cff88b50eb3872565bccbeb6ee15716._comment

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+[[!comment format=mdwn
+ username="http://joeyh.name/"
+ ip="209.250.56.55"
+ subject="comment 3"
+ date="2014-07-04T19:26:00Z"
+ content="""
+Looking at the code, it's pretty clear why this is using a lot of memory:
+
+<pre>
+        fs <- getJournalFiles jl
+        liftIO $ do
+                h <- hashObjectStart g
+                Git.UpdateIndex.streamUpdateIndex g
+                        [genstream dir h fs]
+                hashObjectStop h
+        return $ liftIO $ mapM_ (removeFile . (dir </>)) fs
+</pre>
+
+So the big list in `fs` has to be retained in memory after the files are streamed to update-index, in order for them to be deleted!
+
+Fixing is a bit tricky.. New journal files can appear while this is going on, so it can't just run getJournalFiles a second time to get the files to clean. 
+Maybe delete the file after it's been sent to git-update-index? But git-update-index is going to want to read the file, and we don't really know when it will choose to do so. It could wait a while after we've sent the filename to it, potentially.
+
+Also, per [[!commit 750c4ac6c282d14d19f79e0711f858367da145e4]], we cannot delete the journal files until *after* the commit, or another git-annex process would see inconsistent data!
+
+I actually think I am going to need to use a temp file to hold the list of files..
+"""]]