PaRSEC: check for universal peer access

Check whether thinks that all devices can access the memory of their peers.
If so, we do not need to push modified data back to the host as PaRSEC
will use D2D transfers instead of the host copy. We currently stick
to the owner-computes model so data will always be modified on the same
device and may only be read on a different device.

Signed-off-by: Joseph Schuchart <joseph.schuchart@stonybrook.edu>

ttg/ttg/parsec/ttg.h

@@ -228,6 +228,8 @@ namespace ttg_parsec {
       return im;
     }
 
+    inline bool all_devices_peer_access;
+
   }  // namespace detail
 
   class WorldImpl : public ttg::base::WorldImplBase {
@@ -1075,6 +1077,21 @@ namespace ttg_parsec {
         detail::max_inline_size = inline_size;
       }
     }
+
+    bool all_peer_access = true;
+    /* check whether all GPUs can access all peer GPUs */
+    for (int i = 0; (i < parsec_nb_devices) && all_peer_access; ++i) {
+      parsec_device_module_t *device = parsec_mca_device_get(i);
+      if (PARSEC_DEV_IS_GPU(device->type)) {
+        parsec_device_gpu_module_t *gpu_device = (parsec_device_gpu_module_t*)device;
+        for (int j = 0; (j < parsec_nb_devices) && all_peer_access; ++j) {
+          if (PARSEC_DEV_IS_GPU(device->type)) {
+            all_peer_access &= (gpu_device->peer_access_mask & j);
+          }
+        }
+      }
+    }
+    detail::all_devices_peer_access = all_peer_access;
   }
   inline void ttg_finalize() {
     // We need to notify the current taskpool of termination if we are in user termination detection mode
@@ -3403,9 +3420,14 @@ namespace ttg_parsec {
 
       if constexpr (value_is_const) {
         if (caller->data_flags & detail::ttg_parsec_data_flags::IS_MODIFIED) {
-          /* The data has been modified previously. PaRSEC requires us to pushout
-           * data if we transition from a writer to one or more readers. */
-          need_pushout = true;
+          /* The data has been modified previously. If not all devices can access
+           * their peers then we need to push out to the host so that all devices
+           * have the data available for reading.
+           * NOTE: we currently don't allow users to force the next writer to be
+           *       on a different device. In that case PaRSEC would take the host-side
+           *       copy. If we change our restriction we need to revisit this.
+           *       Ideally, PaRSEC would take the device copy if the owner moves... */
+          need_pushout = !detail::all_devices_peer_access;
         }
 
         /* check for multiple readers */