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zcat vs CamlZip
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Date: 2006-08-29 (19:49)
From: Bárður Árantsson <spam@s...>
Subject: Re: zcat vs CamlZip
Sam Steingold wrote:
> Bardur Arantsson wrote:
>> Sam Steingold wrote:
>>> I read through a huge *.gz file.
>>> I have two versions of the code:
>> [--snip--]
>>> let buf = Buffer.create 1024
>>> let gz_input_line gz_in char_counter line_counter =
>>>   Buffer.clear buf;
>>>   let finish () = incr line_counter; Buffer.contents buf in
>>>   let rec loop () =
>>>     let ch = Gzip.input_char gz_in in
>> This is your most likely culprit. Any kind of "do this for every 
>> character" is usually insanely expensive when you can do it in bulk.
>> (This is especially true when needing to do system calls, or if the 
>> called function cannot be inlined.)
> yes, I thought about it, but I assumed that the ocaml gzip module 
> inlines  Gzip.input_char (obviously the gzip module needs an internal 
> cache so Gzip.input_char does not _always_ translate to a system call, 
> most of the time it just pops a char from the internal buffer).

You can also easily try this in C with fgetc() contrasted with fgets(). 
The difference is _huge_ even if they both do comparable numbers of 
syscalls -- assuming that the buffering is identical (I haven't checked, 
but I think it is a reasonable assumption). In the C case, the inlining 
is not really guaranteed, but I don't think it is in OCaml either -- 
though I honestly don't know. You'd have to check the assembler output 
to see if the call gets inlined.

Inlining aside, memory prefecthing probably also makes a difference in 
favor of reading in bulk and then processing "in bulk".

> at any rate, do you really expect that using Gzip.input and then 
> searching the result for a newline, slicing and dicing to get the 
> individual input lines, &c &c would be faster?

I would guess so, yes.

(There may of course be other reasons for a large portion of the 
difference as others have pointed out.)

Bardur Arantsson

- 'Blackmail' is such an ugly word. I prefer 'extortion'. The X
makes it sound cool.
                                                Bender, 'Futurama'