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| -rw-r--r-- | doc/todo/wishlist:_Provide_a___34__git_annex__34___command_that_will_skip_duplicates/comment_10_d78d79fb2f3713aa69f45d2691cf8dfe._comment | 68 |
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diff --git a/doc/todo/wishlist:_Provide_a___34__git_annex__34___command_that_will_skip_duplicates/comment_10_d78d79fb2f3713aa69f45d2691cf8dfe._comment b/doc/todo/wishlist:_Provide_a___34__git_annex__34___command_that_will_skip_duplicates/comment_10_d78d79fb2f3713aa69f45d2691cf8dfe._comment new file mode 100644 index 000000000..5dbb66cf6 --- /dev/null +++ b/doc/todo/wishlist:_Provide_a___34__git_annex__34___command_that_will_skip_duplicates/comment_10_d78d79fb2f3713aa69f45d2691cf8dfe._comment @@ -0,0 +1,68 @@ +[[!comment format=mdwn + username="http://adamspiers.myopenid.com/" + nickname="Adam" + subject="comment 10" + date="2011-12-23T17:22:11Z" + content=""" +> Your perl script is not O(n). Inserting into perl hash tables has +> overhead of minimum O(n log n). + +What's your source for this assertion? I would expect an amortized +average of `O(1)` per insertion, i.e. `O(n)` for full population. + +> Not counting the overhead of resizing hash tables, +> the grevious slowdown if the bucket size is overcome by data (it +> probably falls back to a linked list or something then), and the +> overhead of traversing the hash tables to get data out. + +None of which necessarily change the algorithmic complexity. However +real benchmarks are far more useful here than complexity analysis, and +[the dangers of premature optimization](http://c2.com/cgi/wiki?PrematureOptimization) +should not be forgotten. + +> Your memory size calculations ignore the overhead of a hash table or +> other data structure to store the data in, which will tend to be +> more than the actual data size it's storing. I estimate your 50 +> million number is off by at least one order of magnitude, and more +> likely two; + +Sure, I was aware of that, but my point still stands. Even 500k keys +per 1GB of RAM does not sound expensive to me. + +> in any case I don't want git-annex to use 1 gb of ram. + +Why not? What's the maximum it should use? 512MB? 256MB? +32MB? I don't see the sense in the author of a program +dictating thresholds which are entirely dependent on the context +in which the program is *run*, not the context in which it's *written*. +That's why systems have files such as `/etc/security/limits.conf`. + +You said you want git-annex to scale to enormous repositories. If you +impose an arbitrary memory restriction such as the above, that means +avoiding implementing *any* kind of functionality which requires `O(n)` +memory or worse. Isn't it reasonable to assume that many users use +git-annex on repositories which are *not* enormous? Even when they do +work with enormous repositories, just like with any other program, +they would naturally expect certain operations to take longer or +become impractical without sufficient RAM. That's why I say that this +restriction amounts to throwing out the baby with the bathwater. +It just means that those who need the functionality would have to +reimplement it themselves, assuming they are able, which is likely +to result in more wheel reinventions. I've already shared +[my implementation](https://github.com/aspiers/git-config/blob/master/bin/git-annex-finddups) +but how many people are likely to find it, let alone get it working? + +> Little known fact: sort(1) will use a temp file as a buffer if too +> much memory is needed to hold the data to sort. + +Interesting. Presumably you are referring to some undocumented +behaviour, rather than `--batch-size` which only applies when merging +multiple files, and not when only sorting STDIN. + +> It's also written in the most efficient language possible and has +> been ruthlessly optimised for 30 years, so I would be very surprised +> if it was not the best choice. + +It's the best choice for sorting. But sorting purely to detect +duplicates is a dismally bad choice. +"""]] |