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
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