Revision 0064fbd0ad69de205ea6ec6999f3d3895e9442c2 authored by Roberto Di Cosmo on 10 January 2012, 20:40:45 UTC, committed by Roberto Di Cosmo on 10 January 2012, 20:40:45 UTC
1 parent b2c3bec
parmap.ml
(**************************************************************************)
(* ParMap: a simple library to perform Map computations on a multi-core *)
(* *)
(* Author(s): Marco Danelutto, Roberto Di Cosmo *)
(* *)
(* This library is free software: you can redistribute it and/or modify *)
(* it under the terms of the GNU Lesser General Public License as *)
(* published by the Free Software Foundation, either version 2 of the *)
(* License, or (at your option) any later version. A special linking *)
(* exception to the GNU Lesser General Public License applies to this *)
(* library, see the LICENSE file for more information. *)
(**************************************************************************)
open ExtLib
(* OS related constants *)
(* a reasonable size for mmapping a file containing even huge result data *)
let huge_size = if Sys.word_size = 64 then 1 lsl 32 else 1 lsl 26;;
(* sequence type, subsuming lists and arrays *)
type 'a sequence = L of 'a list | A of 'a array;;
let debug_enabled = ref false;;
let log_dir = ref (Printf.sprintf "/tmp/.parmap.%d" (Unix.getpid ()))
let debug fmt =
Printf.kprintf (
if !debug_enabled then begin
(fun s -> Format.eprintf "[Parmap]: %s@." s)
end else ignore
) fmt
let info fmt =
Printf.kprintf (fun s -> Format.eprintf "[Parmap]: %s@." s) fmt
(* utils *)
(* would be [? a | a <- startv--endv] using list comprehension from Batteries *)
let ext_intv startv endv =
let s,e = (min startv endv),(max startv endv) in
let rec aux acc = function n -> if n=s then n::acc else aux (n::acc) (n-1)
in aux [] e
;;
(* freopen emulation, from Xavier's suggestion on OCaml mailing list *)
let reopen_out outchan fname =
flush outchan;
if not(Sys.file_exists !log_dir) then Unix.mkdir !log_dir 0o777 ;
let filename = Filename.concat !log_dir fname in
let fd1 = Unix.descr_of_out_channel outchan in
let fd2 = Unix.openfile filename [Unix.O_WRONLY; Unix.O_CREAT; Unix.O_TRUNC] 0o666 in
Unix.dup2 fd2 fd1;
Unix.close fd2
(* unmarshal from a mmap seen as a bigarray *)
let unmarshal fd =
let a = Bigarray.Array1.map_file fd Bigarray.char Bigarray.c_layout true (-1) in
let res = Bytearray.unmarshal a 0 in
Unix.close fd;
res
(* marshal to a mmap seen as a bigarray *)
(* System dependent notes:
- on Linux kernels, we might allocate a mmapped memory area of huge_size and marshal into it directly
let ba = Bigarray.Array1.map_file fd Bigarray.char Bigarray.c_layout true huge_size in
ignore(Bytearray.marshal_to_buffer ba 0 v [Marshal.Closures]);
Unix.close fd
- to be compatible with other systems, notably Mac OS X, which insist in allocating *all*
the declared memory area even for a sparse file, we must choose a less efficient approach:
* marshal the value v to a string s, and compute its size
* allocate a mmap of that exact size,
* copy the string to that mmap
this allocates twice as much memory, and incurs an extra copy of the value v
*)
let marshal fd v =
let s = Marshal.to_string v [Marshal.Closures] in
ignore(Bytearray.mmap_of_string fd s)
(* create a shadow file descriptor *)
let tempfd () =
let name = Filename.temp_file "mmap" "TMP" in
try
let fd = Unix.openfile name [Unix.O_RDWR; Unix.O_CREAT] 0o600 in
Unix.unlink name;
fd
with e -> Unix.unlink name; raise e
(* a simple mapper function that computes 1/nth of the data on each of the n cores in one iteration *)
let simplemapper ncores compute opid al collect =
(* flush everything *)
flush_all();
(* init task parameters *)
let ln = Array.length al in
let chunksize = ln/ncores in
(* create descriptors to mmap *)
let fdarr=Array.init ncores (fun _ -> tempfd()) in
(* call the GC before forking *)
Gc.compact ();
(* spawn children *)
for i = 0 to ncores-1 do
match Unix.fork() with
0 ->
begin
let lo=i*chunksize in
let hi=if i=ncores-1 then ln-1 else (i+1)*chunksize-1 in
let exc_handler e j = (* handle an exception at index j *)
info "error at index j=%d in (%d,%d), chunksize=%d of a total of %d got exception %s on core %d \n%!"
j lo hi chunksize (hi-lo+1) (Printexc.to_string e) i;
exit 1
in
let v = compute al lo hi opid exc_handler in
marshal fdarr.(i) v;
exit 0
end
| -1 -> info "fork error: pid %d; i=%d" (Unix.getpid()) i;
| pid -> ()
done;
(* wait for all children *)
for i = 0 to ncores-1 do
try ignore(Unix.wait())
with Unix.Unix_error (Unix.ECHILD, _, _) -> ()
done;
(* read in all data *)
let res = ref [] in
(* iterate in reverse order, to accumulate in the right order *)
for i = 0 to ncores-1 do
res:= ((unmarshal fdarr.((ncores-1)-i)):'d)::!res;
done;
(* collect all results *)
collect !res
;;
(* a more sophisticated mapper function, with automatic load balancing *)
(* the type of messages exchanged between master and workers *)
type msg_to_master = Ready of int | Error of int * string;;
type msg_to_worker = Finished | Task of int;;
let setup_children_chans oc pipedown fdarr i =
Setcore.setcore i;
(* send stdout and stderr to a file to avoid mixing output from different cores *)
reopen_out stdout (Printf.sprintf "stdout.%d" i);
reopen_out stderr (Printf.sprintf "stderr.%d" i);
(* close the other ends of the pipe and convert my ends to ic/oc *)
Unix.close (snd pipedown.(i));
let pid = Unix.getpid() in
let ic = Unix.in_channel_of_descr (fst pipedown.(i)) in
let receive () = Marshal.from_channel ic in
let signal v = Marshal.to_channel oc v []; flush oc in
let return v =
let d = Unix.gettimeofday() in
let _ = marshal fdarr.(i) v in
debug "worker elapsed %f in marshalling" (Unix.gettimeofday() -. d) in
let finish () =
(debug "shutting down (pid=%d)\n%!" pid;
try close_in ic; close_out oc with _ -> ()
); exit 0 in
receive, signal, return, finish, pid
;;
(* parametric mapper primitive that captures the parallel structure *)
let mapper ncores ~chunksize compute opid al collect =
let ln = Array.length al in
match chunksize with
None -> simplemapper ncores compute opid al collect (* no need of load balancing *)
| Some v when ncores=ln/v -> simplemapper ncores compute opid al collect (* no need of load balancing *)
| Some v ->
(* init task parameters *)
let chunksize = v and ntasks = ln/v in
(* flush everything *)
flush_all ();
(* create descriptors to mmap *)
let fdarr=Array.init ncores (fun _ -> tempfd()) in
(* setup communication channel with the workers *)
let pipedown=Array.init ncores (fun _ -> Unix.pipe ()) in
let pipeup_rd,pipeup_wr=Unix.pipe () in
let oc_up = Unix.out_channel_of_descr pipeup_wr in
(* call the GC before forking *)
Gc.compact ();
(* spawn children *)
for i = 0 to ncores-1 do
match Unix.fork() with
0 ->
begin
let d=Unix.gettimeofday() in
(* primitives for communication *)
Unix.close pipeup_rd;
let receive,signal,return,finish,pid = setup_children_chans oc_up pipedown fdarr i in
let reschunk=ref opid in
let computetask n = (* compute chunk number n *)
let lo=n*chunksize in
let hi=if n=ntasks-1 then ln-1 else (n+1)*chunksize-1 in
let exc_handler e j = (* handle an exception at index j *)
begin
let errmsg = Printexc.to_string e
in info "error at index j=%d in (%d,%d), chunksize=%d of a total of %d got exception %s on core %d \n%!"
j lo hi chunksize (hi-lo+1) errmsg i;
signal (Error (i,errmsg)); finish()
end
in
reschunk:= compute al lo hi !reschunk exc_handler;
info "worker on core %d (pid=%d), segment (%d,%d) of data of length %d, chunksize=%d finished in %f seconds"
i pid lo hi ln chunksize (Unix.gettimeofday() -. d)
in
while true do
(* ask for work until we are finished *)
signal (Ready i);
match receive() with
| Finished -> return (!reschunk:'d); finish ()
| Task n -> computetask n
done;
end
| -1 -> info "fork error: pid %d; i=%d" (Unix.getpid()) i;
| pid -> ()
done;
(* close unused ends of the pipes *)
Array.iter (fun (rfd,_) -> Unix.close rfd) pipedown;
Unix.close pipeup_wr;
(* get ic/oc/wfdl *)
let ocs=Array.init ncores (fun n -> Unix.out_channel_of_descr (snd pipedown.(n))) in
let ic=Unix.in_channel_of_descr pipeup_rd in
(* feed workers until all tasks are finished *)
for i=0 to ntasks-1 do
match Marshal.from_channel ic with
Ready w ->
(debug "sending task %d to worker %d" i w;
let oc = ocs.(w) in
(Marshal.to_channel oc (Task i) []); flush oc)
| Error (core,msg) -> (info "aborting due to exception on core %d: %s" core msg; exit 1)
done;
(* send termination token to all children *)
Array.iter (fun oc ->
Marshal.to_channel oc Finished [];
flush oc;
close_out oc
) ocs;
(* wait for all children to terminate *)
for i = 0 to ncores-1 do
try ignore(Unix.wait())
with Unix.Unix_error (Unix.ECHILD, _, _) -> ()
done;
(* read in all data *)
let res = ref [] in
(* iterate in reverse order, to accumulate in the right order *)
for i = 0 to ncores-1 do
res:= ((unmarshal fdarr.((ncores-1)-i)):'d)::!res;
done;
(* collect all results *)
collect !res
;;
(* the parallel mapfold function *)
let parmapfold ?(ncores=1) ?(chunksize) (f:'a -> 'b) (s:'a sequence) (op:'b->'c->'c) (opid:'c) (concat:'c->'c->'c) : 'c=
(* enforce array to speed up access to the list elements *)
let al = match s with A al -> al | L l -> Array.of_list l in
let compute al lo hi previous exc_handler =
(* iterate in reverse order, to accumulate in the right order *)
let r = ref previous in
for j=0 to (hi-lo) do
try
r := op (f (Array.unsafe_get al (hi-j))) !r;
with e -> exc_handler e j
done; !r
in
mapper ncores ~chunksize compute opid al (fun r -> List.fold_right concat r opid)
;;
(* the parallel map function *)
let parmap ?(ncores=1) ?chunksize (f:'a -> 'b) (s:'a sequence) : 'b list=
(* enforce array to speed up access to the list elements *)
let al = match s with A al -> al | L l -> Array.of_list l in
let compute al lo hi previous exc_handler =
(* iterate in reverse order, to accumulate in the right order, and add to acc *)
let f' j = try f (Array.unsafe_get al (lo+j)) with e -> exc_handler e j in
let rec aux acc =
function
0 -> (f' 0)::acc
| n -> aux ((f' n)::acc) (n-1)
in aux previous (hi-lo)
in
mapper ncores ~chunksize compute [] al (fun r -> ExtLib.List.concat r)
;;
(* the parallel fold function *)
let parfold ?(ncores=1) ?chunksize (op:'a -> 'b -> 'b) (s:'a sequence) (opid:'b) (concat:'b->'b->'b) : 'b=
parmapfold ~ncores ?chunksize (fun x -> x) s op opid concat
;;
(* the parallel map function, on arrays *)
let map_intv lo hi f a =
let l = hi-lo in
if l < 0 then [||] else begin
let r = Array.create (l+1) (f(Array.unsafe_get a lo)) in
for i = 1 to l do
Array.unsafe_set r i (f(Array.unsafe_get a (lo+i)))
done;
r
end
let array_parmap ?(ncores=1) ?chunksize (f:'a -> 'b) (al:'a array) : 'b array=
let compute a lo hi previous exc_handler =
try
Array.concat [(map_intv lo hi f a);previous]
with e -> exc_handler e lo
in
mapper ncores ~chunksize compute [||] al (fun r -> Array.concat r)
;;
(* This code is highly optimised for operations on float arrays:
- knowing in advance the size of the result allows to
pre-allocate it in a shared memory space as a Bigarray;
- to write in the Bigarray memory area using the unsafe
functions for Arrays, we trick the OCaml compiler into
using the Bigarray memory as an Array as follows
Array.unsafe_get (Obj.magic arr_out) 1
This works because OCaml compiles access to float arrays
as unboxed data, without further integrity checks;
- the final copy into a real OCaml array is done via a memcpy in C.
This approach gives a performance which is 2 to 3 times higher
w.r.t. array_parmap, at the price of using Obj.magic and
knowledge on the internal representation of arrays and bigarrays.
*)
exception WrongArraySize
type buf= (float, Bigarray.float64_elt, Bigarray.c_layout) Bigarray.Array1.t * int;; (* should be a long int some day *)
let init_shared_buffer a =
let size = Array.length a in
let fd = tempfd() in
let arr = Bigarray.Array1.map_file fd Bigarray.float64 Bigarray.c_layout true size in
(* The mmap() function shall add an extra reference to the file associated
with the file descriptor fildes which is not removed by a subsequent close()
on that file descriptor.
http://pubs.opengroup.org/onlinepubs/009695399/functions/mmap.html
*)
Unix.close fd; (arr,size)
;;
let array_float_parmap ?(ncores=1) ?chunksize ?result ?sharedbuffer (f:'a -> float) (al:'a array) : float array =
let size = Array.length al in
let barr_out =
match sharedbuffer with
Some (arr,s) ->
if s<size then
(info "shared buffer is too small to hold the input in array_float_parmap"; raise WrongArraySize)
else arr
| None -> fst (init_shared_buffer al)
in
(* trick the compiler into accessing the Bigarray memory area as a float array:
the data in Bigarray is placed at offset 1 w.r.t. a normal array, so we
get a pointer to that zone into arr_out_as_array, and have it typed as a float
array *)
let barr_out_as_array = Array.unsafe_get (Obj.magic barr_out) 1 in
let compute _ lo hi _ exc_handler =
try
for i=lo to hi do
Array.unsafe_set barr_out_as_array i (f (Array.unsafe_get al i))
done
with e -> exc_handler e lo
in
mapper ncores ~chunksize compute () al (fun r -> ());
let res =
match result with
None -> Bytearray.to_floatarray barr_out size
| Some a ->
if Array.length a < size then
(info "result array is too small to hold the result in array_float_parmap"; raise WrongArraySize)
else
Bytearray.to_this_floatarray a barr_out size
in res
;;
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