authors: Jason Lowe-Power
last edited: 2023-01-11 16:05:55 +0000

Introduction to Ruby

Ruby comes from the multifacet GEMS project. Ruby provides a detailed cache memory and cache coherence models as well as a detailed network model (Garnet).

Ruby is flexible. It can model many different kinds of coherence implementations, including broadcast, directory, token, region-based coherence, and is simple to extend to new coherence models.

Ruby is a mostly drop-in replacement for the classic memory system. There are interfaces between the classic gem5 MemObjects and Ruby, but for the most part, the classic caches and Ruby are not compatible.

In this part of the book, we will first go through creating an example protocol from the protocol description to debugging and running the protocol.

Before diving into a protocol, we will first talk about some of the architecture of Ruby. The most important structure in Ruby is the controller, or state machine. Controllers are implemented by writing a SLICC state machine file.

SLICC is a domain-specific language (Specification Language including Cache Coherence) for specifying coherence protocols. SLICC files end in “.sm” because they are state machine files. Each file describes states, transitions from a begin to an end state on some event, and actions to take during the transition.

Each coherence protocol is made up of multiple SLICC state machine files. These files are compiled with the SLICC compiler which is written in Python and part of the gem5 source. The SLICC compiler takes the state machine files and output a set of C++ files that are compiled with all of gem5’s other files. These files include the SimObject declaration file as well as implementation files for SimObjects and other C++ objects.

Currently, gem5 supports compiling only a single coherence protocol at a time. For instance, you can compile MI_example into gem5 (the default, poor performance, protocol), or you can use MESI_Two_Level. But, to use MESI_Two_Level, you have to recompile gem5 so the SLICC compiler can generate the correct files for the protocol. We discuss this further in the compilation section <MSI-building-section>

Now, let’s dive into implementing our first coherence protocol!