I'm in the process of adding a Data.Sequence.Unboxed to unboxed-containers.
I hope to have it in hackage today or tomorrow, should its performance work
out as well as Data.Set.Unboxed.
Be warned the API will likely be shifting on you for a little while, while I
figure out a better way to manage all of the instances.
-Edward Kmett
On Tue, Apr 7, 2009 at 7:49 AM, Manlio Perillo
Hi.
I'm still working on my Netflix Prize project.
For a function I wrote, I really need a data structure that is both space efficient (unboxed elements) and with an efficient snoc operation.
I have pasted a self contained module with the definition of the function I'm using: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=3453
The movie ratings are loaded from serialized data, and the result is serialized again, using the binary package:
transcodeIO :: IO () transcodeIO = do input <- L.hGetContents stdin let output = encodeZ $ transcode $ decodeZ input L.hPut stdout output
(here encodeZ and decodeZ are wrappers around Data.Binary.encode and Data.Binary.decode, with support to gzip compression/decompression)
This function (transcodeIO, not transcode) takes, on my Core2 CPU T7200 @ 2.00GHz:
real 30m8.794s user 29m30.659s sys 0m10.313s
1068 Mb total memory in use
The problem here is with snocU, that requires a lot of copying.
I rewrote the transcode function so that the input data set is split in N parts: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=3453#a3456
The mapReduce function is the one defined in the Real World Haskell.
The new function takes (using only one thread):
real 18m48.039s user 18m30.901s sys 0m6.520s
1351 Mb total memory in use
The additional required memory is probably caused by unionsWith, that is not strict. The function takes less time, since array copying is optimized. I still use snocU, but on small arrays.
GC time is very high: 54.4%
Unfortunately I can not test with more then one thread, since I get segmentation faults (probably a bug with uvector packages).
I also got two strange errors (but this may be just the result of the segmentation fault, I'm not able to reproduce them):
tpp.c:63: __pthread_tpp_change_priority: Assertion `new_prio == -1 || (new_prio >= __sched_fifo_min_prio && new_prio <= __sched_fifo_max_prio)' failed.
internal error: removeThreadFromQueue: not found (GHC version 6.8.2 for i386_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
Now the question is: what data structure should I use to optimize the transcode function?
IMHO there are two solutions:
1) Use a "lazy" array. Something like ByteString.Lazy, and what is available in storablevector package.
Using this data structure, I can avoid the use of appendU.
2) Use an "unboxed" list.
Something like: http://mdounin.ru/hg/nginx-vendor-current/file/tip/src/core/ngx_list.h
That is: a linked list of unboxed arrays, but unlike the lazy array solution, a snoc operation avoid copying if there is space in the current array.
I don't know if this is easy/efficient to implement in Haskell.
Any other suggestions?
Thanks Manlio Perillo _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe