Synchronization

Per Daniele

The nu+ manycore supports an hardware mechanism for Synchronization. The mechanism of synchronization is based on Barrier primitive. A barrier for a group of threads in the source code means any thread must stop at this point and cannot proceed until all other threads reach this barrier. The mechanism of Barrier Synchronization implemented in this system is based on master-slave architecture, with master distributed on tiles. We support more independent barriers in parallel execution on manycore in each time. For identifying the different barriers we have an ID for each Barrier. Each Synchronization Core Master handles a range of Barrier in manycore.

Barrier Synchronization Protocol

The Barrier protocol is based on a message passing. We have three type of message:

• Setup_Message: It configures the struct of Barrier in Synchronization Core;
• Account_Message: It notifies to Synchronization Core that the thread is arrived to point of Barrier;
• Release_Message: It notifies from Synchronization Core to Barrier Core that all threads are arrived to point of Synchronization, so the Barrier can end.

In this protocol the Setup_Message and Account_Message are out-order and we use an enable bit for managing the Setup. The size of message change with number of Barrier Synchronization supported, as view in figure:

The size of each message is of 1 flit (64 bit), and the field size of message change with number of Barrier supported. We have these fields in the messages:

• Type: It specifies the type of message: '00'(Setup_Message), '01'(Account_Message), '10'(Release_message). The size is 2 bits;
• Counter': It specifies the number of Hardware Threads that takes part in the Barrier Synchronization. The size is on N bits, where N is log2 of total number of Hardware Threads in the manycore;
• Barrier ID: It identifies the Barrier. The size si on M bits, where M is the log2 of total number of Barrier supported by architecture; it is defined in the file(indicare il file).
• Tile Source: It specifies the ID of Tile Source of Barrier. We use it for destination multicast of Release Message. The size is T and it is log2 of total number of tiles in the manycore.