GNAT implementation of socket sets

GNAT implementation of socket sets

[Dmitry A. Kazakov]

GNAT.Sockets implementation of Socket_Set_Type has no operation to walk 
that set by position. It seemed not a big issue until I noticed that the 
implementation of

    procedure Get (Item : in out Socket_Set_Type; Socket : out Socket_Type);
    --  Extract a Socket from socket set Item. Socket is set to
    --  No_Socket when the set is empty.

seems to always return the last element of the set.

If so, it imposes a severe problem for an application that deals with 
multiple non-blocking sockets, because it prioritizes connections with 
sockets with greater file numbers. Under heavy load the application will 
serve only one connection.

Typically such issues are resolved by a round-robin schema to balance 
the load. But that requires walking the set and extracting an element at 
a position.

Can anybody confirm that and suggest a workaround?

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: GNAT implementation of socket sets

[Simon Wright]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> GNAT.Sockets implementation of Socket_Set_Type has no operation to
> walk that set by position. It seemed not a big issue until I noticed
> that the implementation of
>
>    procedure Get (Item : in out Socket_Set_Type; Socket : out Socket_Type);
>    --  Extract a Socket from socket set Item. Socket is set to
>    --  No_Socket when the set is empty.
>
> seems to always return the last element of the set.

Indeed.

Implemented as:

   /* Get last socket and remove it from the socket set SET.  LAST is the
      maximum value of the largest socket.  This hint is used to avoid scanning
      very large socket sets.  On return, LAST is set to the actual largest
      socket in the socket set. */

   void
   __gnat_get_socket_from_set (fd_set *set, int *last, int *socket)
   {
     *socket = *last;
     FD_CLR (*socket, set);
     __gnat_last_socket_in_set (set, last);
   }

where

   void
   __gnat_last_socket_in_set (fd_set *set, int *last)
   {
     int s;
     int l;
     l = -1;

   #ifdef _WIN32
     /* More efficient method for NT. */
     for (s = 0; s < set->fd_count; s++)
       if ((int) set->fd_array[s] > l)
         l = set->fd_array[s];

   #else

     for (s = *last; s != -1; s--)
       if (FD_ISSET (s, set))
         {
           l = s;
           break;
         }
   #endif

     *last = l;
   }