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+`git-annex unused` has to compare large sets of data
+(all keys with content present in the repository,
+with all keys used by files in the repository), and so
+uses more memory than git-annex typically needs; around
+60-80 mb when run in a repository with 80 thousand files.
+
+I would like to reduce this. One idea is to use a bloom filter. 
+For example, construct a bloom filter of all keys used by files in
+the repository. Then for each key with content present, check if it's
+in the bloom filter. Since there can be false negatives, this might
+miss finding some unused keys. The probability/size of filter
+could be tunable.
+
+Another way might be to scan the git log for files that got removed
+or changed what key they pointed to. Correlate with keys with content
+currently present in the repository (possibly using a bloom filter again),
+and that would yield a shortlist of keys that are probably not used. 
+Then scan thru all files in the repo to make sure that none point to keys
+on the shortlist.
+
+----
+
+`git annex unused --from remote` is much worse, using hundreds of mb of
+memory. It has not been profiled at all yet, and can probably be improved
+somewhat by fixing whatever memory leak it (probably) has.