[Haskell-cafe] advice on space efficient data structure with efficient snoc operation

Manlio Perillo manlio_perillo at libero.it
Tue Apr 7 07:49:56 EDT 2009


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:

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:

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:

    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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