Comment author: administrator

[Timing the Unix.select function]
i do compile this on aix and linux with
ocamlopt -o messe unix.cmxa mess1.ml
then i get on aix:
time messe 1000 0.0
start: 1008862247.689975977
end: 1008862259.975527048

real 0m12.318s
user 0m5.350s
sys 0m0.070s
and on linux

time messe 1000 0.0
start: 1008862258.931671977
end: 1008862259.318706036

real 0m0.406s
user 0m0.370s
sys 0m0.040s
which is much faster,

I don't think this speed difference has anything to do with OCaml.
Unix.select is a thin wrapper around the select() system call, and the
code is OS-independent. Chances are that if you rewrite your test in
C, you'll see the same speed difference. Some kernels are simply more
efficient (better algorithms and data structures) than others...

• Xavier Leroy

Comment author: administrator

On Fri, 4 Jan 2002, Xavier Leroy wrote:

I don't think this speed difference has anything to do with OCaml.
Unix.select is a thin wrapper around the select() system call, and the
code is OS-independent. Chances are that if you rewrite your test in
C, you'll see the same speed difference.

i think the problem is the use of FD_SETSIZE in select
and in fdset_to_fdlist:

the value of FD_SETSIZE on aix is
/usr/include/sys/time.h: #define FD_SETSIZE 32767

so you run through the for-loop in fdset_to_fdlist  32767 times !

anyway, i did make some changes in select.c and my testprogramm
runs good

time mess 1000 0.001
start (pid=44244): 1010376716.065811992
end: 1010376717.095213056

real 0m1.036s
user 0m0.020s
sys 0m0.030s

but i did not test anything else!

-- m.becker

*** select.c Mon Jan 7 04:53:14 2002
--- o.select.c Mon Jan 7 04:38:55 2002

*** 33,63 ****

typedef fd_set file_descr_set;

! static int fdlist_to_fdset(value fdlist, file_descr_set *fdset)
{
! int maxi;
! int fdd;
!
! value l;
!
! FD_ZERO(fdset);
! maxi = 0;
!
for (l = fdlist; l != Val_int(0); l = Field(l, 1)) {
! fdd = Int_val(Field(l, 0)) ;
! maxi = fdd < maxi ? maxi : fdd ;
! FD_SET(fdd, fdset);
}

• return maxi;
  }

! static value fdset_to_fdlist(file_descr_set *fdset, int maxi)
{
int i;
value res = Val_int(0);

Begin_root(res);

! for (i = maxi; i >= 0; i--) {
if (FD_ISSET(i, fdset)) {
value newres = alloc_small(2, 0);
Field(newres, 0) = Val_int(i);
--- 33,54 ----

typedef fd_set file_descr_set;

! static void fdlist_to_fdset(value fdlist, file_descr_set *fdset)
{
! value l;
! FD_ZERO(fdset);
for (l = fdlist; l != Val_int(0); l = Field(l, 1)) {
! FD_SET(Int_val(Field(l, 0)), fdset);
}
}

! static value fdset_to_fdlist(file_descr_set *fdset)
{
int i;
value res = Val_int(0);

Begin_root(res);

! for (i = FD_SETSIZE - 1; i >= 0; i--) {
if (FD_ISSET(i, fdset)) {
value newres = alloc_small(2, 0);
Field(newres, 0) = Val_int(i);

*** 71,78 ****

CAMLprim value unix_select(value readfds, value writefds, value exceptfds, value timeout)
{

• int cc, maxi;
  

• file_descr_set read, write, except;
  double tm;
  struct timeval tv;
  --- 62,67 ----

*** 82,96 ****
value read_list = Val_unit, write_list = Val_unit, except_list = Val_unit;

Begin_roots3 (read_list, write_list, except_list);

!
! maxi = fdlist_to_fdset(readfds, &read);
!
! cc = fdlist_to_fdset(writefds, &write);
! maxi = cc < maxi ? maxi : cc;
!
! cc = fdlist_to_fdset(exceptfds, &except);
! maxi = cc < maxi ? maxi : cc;
!
tm = Double_val(timeout);
if (tm < 0.0)
tvp = (struct timeval *) NULL;
--- 71,79 ----
value read_list = Val_unit, write_list = Val_unit, except_list = Val_unit;

Begin_roots3 (read_list, write_list, except_list);

! fdlist_to_fdset(readfds, &read);
! fdlist_to_fdset(writefds, &write);
! fdlist_to_fdset(exceptfds, &except);
tm = Double_val(timeout);
if (tm < 0.0)
tvp = (struct timeval *) NULL;

*** 100,111 ****
tvp = &tv;
}
enter_blocking_section();
! retcode = select(maxi + 1, &read, &write, &except, tvp);
leave_blocking_section();
if (retcode == -1) uerror("select", Nothing);
! read_list = fdset_to_fdlist(&read, maxi);
! write_list = fdset_to_fdlist(&write, maxi);
! except_list = fdset_to_fdlist(&except, maxi);
res = alloc_small(3, 0);
Field(res, 0) = read_list;
Field(res, 1) = write_list;
--- 83,94 ----
tvp = &tv;
}
enter_blocking_section();
! retcode = select(FD_SETSIZE, &read, &write, &except, tvp);
leave_blocking_section();
if (retcode == -1) uerror("select", Nothing);
! read_list = fdset_to_fdlist(&read);
! write_list = fdset_to_fdlist(&write);
! except_list = fdset_to_fdlist(&except);
res = alloc_small(3, 0);
Field(res, 0) = read_list;
Field(res, 1) = write_list;

Comment author: administrator

i think the problem is the use of FD_SETSIZE in select
and in fdset_to_fdlist:
the value of FD_SETSIZE on aix is
/usr/include/sys/time.h: #define FD_SETSIZE 32767
so you run through the for-loop in fdset_to_fdlist 32767 times !

OK, I overlooked this fact :-( Thanks for proving me wrong. I agree
a fix is in order, probably along the lines of what you sent me.

• Xavier Leroy

Comment author: administrator

Implemented more efficient building of return FD lists. XL, 2002-05-06