Re: [Caml-list] Threads performance issue.

["Rémi Dewitte" <remi@gide.net>]

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Yaron,

I use a slightly modified version of the CSV library's load_rows . Here is
the main code which is highly imperative style. I might transform it in
purely functional style ?

The main program is :

open Printf;;
open Sys;;
let timed_exec start_message f =
  print_string start_message;
  let st1 = time () in
  let r = f () in
  print_endline ("done in " ^ (string_of_float ((time ()) -. st1)) );
  r;;

(* This line enabled makes the program really slow ! *)
let run_threaded f = Thread.create (fun () -> f (); Thread.exit ()) ()

let () = timed_exec "Reading data " (fun () ->
  load_rows (fun _ -> ()) (open_in "file1.csv");
  load_rows (fun _ -> ()) (open_in "file2.csv");
  ()
)

The load_rows :
let load_rows ?(separator = ',') ?(nread = -1) f chan =
  let nr = ref 0 in
  let row = ref [] in            (* Current row. *)
  let field = ref [] in            (* Current field. *)
  let state = ref StartField in        (* Current state. *)
  let end_of_field () =
    let field_list = List.rev !field in
    let field_len = List.length field_list in
    let field_str = String.create field_len in
    let rec loop i = function
    [] -> ()
      | x :: xs ->
      field_str.[i] <- x;
      loop (i+1) xs
    in
    loop 0 field_list;
    row := (Some field_str) :: !row;
    field := [];
    state := StartField
  in
  let empty_field () =
    row := None :: !row;
    field := [];
    state := StartField
  in
  let end_of_row () =
    let row_list = List.rev !row in
    f row_list;
    row := [];
    state := StartField;
    nr := !nr + 1;
  in
  let rec loop () =
    let c = input_char chan in
    if c != '\r' then (            (* Always ignore \r characters. *)
      match !state with
      StartField ->            (* Expecting quote or other char. *)
        if c = '"' then (
          state := InQuotedField;
          field := []
        ) else if c = separator then (* Empty field. *)
          empty_field ()
        else if c = '\n' then (    (* Empty field, end of row. *)
          empty_field ();
          end_of_row ()
        ) else (
          state := InUnquotedField;
          field := [c]
        )
    | InUnquotedField ->        (* Reading chars to end of field. *)
        if c = separator then    (* End of field. *)
          end_of_field ()
        else if c = '\n' then (    (* End of field and end of row. *)
          end_of_field ();
          end_of_row ()
        ) else
          field := c :: !field
    | InQuotedField ->        (* Reading chars to end of field. *)
        if c = '"' then
          state := InQuotedFieldAfterQuote
        else
          field := c :: !field
    | InQuotedFieldAfterQuote ->
        if c = '"' then (        (* Doubled quote. *)
          field := c :: !field;
          state := InQuotedField
        ) else if c = '0' then (    (* Quote-0 is ASCII NUL. *)
          field := '\000' :: !field;
          state := InQuotedField
        ) else if c = separator then (* End of field. *)
          end_of_field ()
        else if c = '\n' then (    (* End of field and end of row. *)
          end_of_field ();
          end_of_row ()
        ) else (            (* Bad single quote in field. *)
          field := c :: '"' :: !field;
          state := InQuotedField
        )
    ); (* end of match *)
  if( nread < 0 or !nr < nread) then loop () else ()
  in
  try
    loop ()
  with
      End_of_file ->
    (* Any part left to write out? *)
    (match !state with
         StartField ->
           if !row <> [] then
         ( empty_field (); end_of_row () )
       | InUnquotedField | InQuotedFieldAfterQuote ->
           end_of_field (); end_of_row ()
       | InQuotedField ->
           raise (Bad_CSV_file "Missing end quote after quoted field.")
    )


Thanks,
Rémi

On Mon, Feb 16, 2009 at 17:47, Yaron Minsky <yminsky@gmail.com> wrote:

> 2009/2/16 Rémi Dewitte <remi@gide.net>
>
>> Hello,
>>
>> I would like to read two files in two different threads.
>>
>> I have made a first version reading the first then the second and it takes
>> 2.8s (native).
>>
>> I decided to make a threaded version and before any use of thread I
>> realized that just linking no even using it to the threads library makes my
>> first version of the program to run in 12s !
>
>
> Do you have a short benchmark you can post?  The idea that the
> thread-overhead would make a difference like that, particularly for IO-bound
> code (which I'm guessing this is) is pretty surprising.
>
> y
>
>
>>
>> I use pcre, extlib, csv libraries as well.
>>
>> I guess it might come from GC slowing down thinks here, doesn't it ? Where
>> can it come from otherwise ? Is there a workaround or something I should
>> know ?
>>
>> Can ocaml use multiple cores ?
>>
>> Do you have few pointers on libraries to make parallel I/Os ?
>>
>> Thanks,
>> Rémi
>>
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>

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