in reply to Fast parsing for cypher block chaining

Why not let Perl do the buffering for you. read and write are buffered (unlike sysread and syswrite). 
while (my $bufsize = read(INFILE, my $eight_byte_chunk, 8)) {
   ...[ Add padding    ]...
   ...[ Encrypt        ]...
   ...[ Merge with IV  ]...
   ...[ Compute new IV ]...
   write(OUTFILE, $eight_byte_chunk, 8);
}

[download]

Replies are listed 'Best First'.
Re^2: Fast parsing for cypher block chaining
by fluffyvoidwarrior (Monk) on Feb 28, 2006 at 18:26 UTC
    Because I thought read would be slower than sysread and the 8 byte buffer would result in a lot of disk thrashing compared to 64k buffer
[reply]
      the 8 byte buffer would result in a lot of disk thrashing compared to 64k buffer

      And there shouldn't be any disk trashing because read and write are buffered. The size of the buffer isn't 8 bytes as you claim, but rather a multiple of the size of a disc sector. The disk is not accessed every time read and write is called.

      Because I thought read would be slower than sysread

      sysread + syswrite + manual buffering in Perl
      should be slower than
      read + write + well tested buffering in C

      Note that I didn't say my way is faster, just that it might be. Benchmark to find out which is faster on your system.

[reply]
[d/l]
[select]
        Just benchmarked a comparison between sysread/write with substr() and straight perl IO read/write if anyone is still interested.
        The results are a bit of a shock!
        Parsing a 700Mb file in 8 byte chunks took 81 seconds with the sysread method. Using perl buffered read/write it took 522 seconds. It seems sysread and handling your own buffering can produce performance gains of upto 700% - which is what I'm looking for.
        Heres my code just in case I've done anything dumb (I'm assuming using OOP IO is OK)
              $infile = new IO::File;
      $outfile = new IO::File;
      
      $infile->open($input_filepath);
      $outfile->open(">$output_filepath");

      for($chunk_counter = 0; $chunk_counter <  $infile_num_chunks +1;
+ $chunk_counter = $chunk_counter + 1){
        $infile->read($buffer, 8);
        $outfile->write($buffer,8);
      }

        [download]

        I originally used a while construct for loop control but then thought maybe it was slowing things down. It was. Using "while" took 522 secs. Using the counter as above took 449 secs. Either way sysread and substr() is loads faster.
[reply]
[d/l]

In Section Seekers of Perl Wisdom