B.4.2 Application Based Invalidations B.1 First Access to a Page B.2 Trying to Read an Initialized Page B.3 Trying to Write to an Initialized Page B.4 Updating the Read Only Pages

As a result, the thread can access any item on the invalidated pages. A page fault will occur and a new copy of the page will be fetched to him. Notice, that every time the weak memory manager processes a page fault, it takes a new copy of the page from the owner of the writable copy (since the memory manager does not save pages by itself). Remember, that as we do not allow multiple writable copies of the same page, this copy always contains the newest values of all data. Several messages in the above dialog are optional. For instance, if a thread asks to update pages which are not stored on the host at all, or which are present as writable, then Messages 4 and 5 will be unnecessary. In any case, Messages 3 and 6 must be sent, since only the memory manager knows the exact distribution of all pages in the system. As we show further, any thread could update not only pages on its own host, but also pages located on other nodes in the system. >From a ParC user program point of view, this method is preferable when any thread may possibly update some data and would like to inform all the others about it. In this case, Message 4 will be sent to all the hosts that possess read only copies of that data, and the memory manager will wait for connrmation from all of them (Message 5). Several memory sync functions are currently implemented. Recall that activating these functions is only meaningful under the weak memory manager. However, since a ParC user can experiment with the system and his ParC program using various types of memory management and load balancing protocols, we decided to allow the usage of memory sync functions under other memory managers as well. Therefore, if a user who chooses a DSM algorithm other than the weak forgets to remove the memory synchronization functions from the program code, the program will compile and run without error messages or warnings. Instead, only Messages 1 and 8 (Figure 21) will be sent in this case. Millipede can automatically recognize the type of memory manager used during the memory sync function call, and can therefore immediately return control to the thread without sending any further message to the memory manager. We have implemented the following functions: total mem sync() A very heavy …