Page Comparison

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• Client-side aggregation: The client-side aggregation is implemented in a way that a client requests a number of objects one by one where each object contains one integer value. Consequently, a read-RPC gets invoked for every object and the client locally computes the sum.
• Server-side aggregation via hashtable lookup: A range of keys is passed to the server and the server performs a lookup in its own hash table for every object. Again, each object contains a single integer which gets added up (as shown in Listing 1). Once the aggregation is done, the resulting sum is sent back to the server via RPC.
• Server-side aggregation via hashTable forEach: The hash table in the MasterServer offers a forEach method that iterates over all objects contained in the hash table. A callback can be registered to that method which is shown in Listing 2.
• Server-side aggregation via Log traversal: In a MasterServer in RAMCloud, the actual objects are stored in a log. In this experiment, the complete Log is traversed without using the hash table at all (as shown in Listing 3).

for(uint64_t i = 0; i < range; ++i)
   {
        LogEntryHandle handle = objectMap.lookup(tableId, i);
        const Object* obj = handle->userData<Object>();

        int *p;
        p = (int*) obj->data;
        sum += (uint64_t)*p;
   }

/**
* Aggregation Callback
*/
void
aggregateCallback(LogEntryHandle handle, uint8_t type,
                      void *cookie)
{
        const Object* obj = handle->userData<Object>();
        MasterServer *server = reinterpret_cast<MasterServer*>(cookie);

        int *p;
        p = (int*) obj->data;
        server->sum += (uint64_t)*p;
}

//**
  * Aggregation via traversing all SegmentEntries throughout the entire Log
  */
uint64_t
Log::aggregate()
{
  uint64_t sum = 0;

  //Iterate over all Segments in the Log
  foreach (ActiveIdMap::value_type& idSegmentPair, activeIdMap) {
        Segment* segment = idSegmentPair.second;

        //Iterate over all SegmentEntries in a Segment
        for (SegmentIterator i(segment); !i.isDone(); i.next()) {
                SegmentEntryHandle seh = i.getHandle();
                //Check that it is an Object
                if (seh->type()==560620143) {
                        const Object* obj = seh->userData<Object>();
                        int *p;
                        p = (int*) obj->data;
                        sum += *p;
                }
        }
   }
   return sum;
}

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