Global Operations

global operations

P4 supports a number of operations for dealing with all processes at once.

p4_broadcastx
\begin{example} p4_broadcastx(type, data, data_len, data_type) int type; char *data; int data_len, data_type; \end{example}

p4_broadcast
\begin{example} p4_broadcast(type, data, data_len) int type; char *data; int data_len; \end{example}
provide the ability to broadcast messages like p4_send and p4_sendx.

p4_global_op
\begin{example} p4_global_op(type,x,nelem,size,op,data_type) int type; char *x; int size, nelem; int (*op)(); int data_type; \end{example}
where op is one of:
\begin{example} p4_int_absmax_op() p4_int_absmin_op() p4_int_max_op() p4_int_min... ...) p4_flt_max_op() p4_flt_min_op() p4_flt_mult_op() p4_flt_sum_op() \end{example}

and data_type is one of P4INT, P4LNG, P4FLT, or P4DBL.

This collection of routines provide the ability to do a variety of global operations. See the example programs in subdirectory p4/contrib. They apply the commutative operation op globally to x on an element-by-element basis and broadcast the result to all nodes. That is each process ends up with


\begin{example} for (i=0; i<n; i++) x[i] = x[node 0][i] op x[node 1][i] op x[node 2][i] op ... \end{example}
op should be of the form


\begin{example} VOID op(char *x, char *y, int nelem) \{ data_type *a = (data_... ...= (data_type *) y; \par while (nelem--) *a++ operation= *b++; \} \end{example}
where data_type and operation are chosen appropriately.

The order in which nodes apply the operation is undefined (hence op must be commutative). The communication may be internally sub-blocked so the function op should not be hardwired to specific vector lengths.

This is still a relatively primitive version, which gathers the necessary data up a balanced binary tree and then uses p4_broadcast to send the results back.

barrier p4_global_barrier
\begin{example} VOID p4_global_barrier(type) int type; \end{example}
This procedure takes one argument which is the message type to be used for internal message-passing. It causes the invoking process to hang until all processes specified in the procgroup file have invoked the procedure.

Functions for Shared Memory,Functions for Timing p4 Programs,Functions for Message Passing,Top