last edited: 2023-11-20 18:18:55 +0000
A configuration script for the MSI protocol
First, create a new configuration directory in
configs/. Just like all
gem5 configuration files, we will have a configuration run script. For
the run script, we can start with
simple-config-chapter. Copy this file to
simple_ruby.py in your new
We will make a couple of small changes to this file to use Ruby instead of directly connecting the CPU to the memory controllers.
First, so we can test our coherence protocol, let’s use two CPUs.
system.cpu = [X86TimingSimpleCPU(), X86TimingSimpleCPU()]
Next, after the memory controllers have been instantiated, we are going to create the cache system and set up all of the caches. Add the following lines after the CPU interrupts have been created, but before instantiating the system.
system.caches = MyCacheSystem() system.caches.setup(system, system.cpu, [system.mem_ctrl])
Like the classic cache example in cache-config-chapter, we are going to
create a second file that contains the cache configuration code. In this
file we are going to have a class called
MyCacheSystem and we will
setup function that takes as parameters the CPUs in the
system and the memory controllers.
You can download the complete run script here.
Cache system configuration
Now, let’s create a file
msi_caches.py. In this file, we will create
MyCacheSystem which will inherit from
Directory which will inherit from the SimObjects created
by SLICC from our two state machines, and
MyNetwork which will inherit
Let’s start with the
L1Cache. First, we will inherit from
L1Cache_Controller since we named our L1 cache “L1Cache” in the state
machine file. We also include a special class variable and class method
for tracking the “version number”. For each SLICC state machine, you
have to number them in ascending order from 0. Each machine of the same
type should have a unique version number. This is used to differentiate
the individual machines. (Hopefully, in the future this requirement will
class L1Cache(L1Cache_Controller): _version = 0 @classmethod def versionCount(cls): cls._version += 1 # Use count for this particular type return cls._version - 1
Next, we implement the constructor for the class.
def __init__(self, system, ruby_system, cpu): super(L1Cache, self).__init__() self.version = self.versionCount() self.cacheMemory = RubyCache(size = '16kB', assoc = 8, start_index_bit = self.getBlockSizeBits(system)) self.clk_domain = cpu.clk_domain self.send_evictions = self.sendEvicts(cpu) self.ruby_system = ruby_system self.connectQueues(ruby_system)
We need the CPUs in this function to grab the clock domain and system is
needed for the cache block size. Here, we set all of the parameters that
we named in the state machine file (e.g.,
cacheMemory). We will set
sequencer later. We also hardcode the size an associativity of the
cache. You could add command line parameters for these options, if it is
important to vary them at runtime.
Next, we implement a couple of helper functions. First, we need to figure out how many bits of the address to use for indexing into the cache, which is a simple log operation. We also need to decide whether to send eviction notices to the CPU. Only if we are using the out-of-order CPU and using x86 or ARM ISA should we forward evictions.
def getBlockSizeBits(self, system): bits = int(math.log(system.cache_line_size, 2)) if 2**bits != system.cache_line_size.value: panic("Cache line size not a power of 2!") return bits def sendEvicts(self, cpu): """True if the CPU model or ISA requires sending evictions from caches to the CPU. Three scenarios warrant forwarding evictions to the CPU: 1. The O3 model must keep the LSQ coherent with the caches 2. The x86 mwait instruction is built on top of coherence 3. The local exclusive monitor in ARM systems """ return True
Finally, we need to implement
connectQueues to connect all of the
message buffers to the Ruby network. First, we create a message buffer
for the mandatory queue. Since this is an L1 cache and it will have a
sequencer, we need to instantiate this special message buffer. Next, we
instantiate a message buffer for each buffer in the controller. All of
the “to” buffers we must set the “master” to the network (i.e., the
buffer will send messages into the network), and all of the “from”
buffers we must set the “slave” to the network. These names are the
same as the gem5 ports, but message buffers are not currently
implemented as gem5 ports. In this protocol, we are assuming the
message buffers are ordered for simplicity.
def connectQueues(self, ruby_system): self.mandatoryQueue = MessageBuffer() self.requestToDir = MessageBuffer(ordered = True) self.requestToDir.master = ruby_system.network.slave self.responseToDirOrSibling = MessageBuffer(ordered = True) self.responseToDirOrSibling.master = ruby_system.network.slave self.forwardFromDir = MessageBuffer(ordered = True) self.forwardFromDir.slave = ruby_system.network.master self.responseFromDirOrSibling = MessageBuffer(ordered = True) self.responseFromDirOrSibling.slave = ruby_system.network.master
Now, we can similarly implement the directory. There are three
differences from the L1 cache. First, we need to set the address ranges
for the directory. Since each directory corresponds to a particular
memory controller for a subset of the address range (possibly), we need
to make sure the ranges match. The default address ranges for Ruby
Next, we need to set the master port
memory. This is the port that
sends messages when
queueMemoryRead/Write is called in the SLICC code.
We set it the to the memory controller port. Similarly, in
connectQueues we need to instantiate the special message buffer
responseFromMemory like the
mandatoryQueue in the L1 cache.
class DirController(Directory_Controller): _version = 0 @classmethod def versionCount(cls): cls._version += 1 # Use count for this particular type return cls._version - 1 def __init__(self, ruby_system, ranges, mem_ctrls): """ranges are the memory ranges assigned to this controller. """ if len(mem_ctrls) > 1: panic("This cache system can only be connected to one mem ctrl") super(DirController, self).__init__() self.version = self.versionCount() self.addr_ranges = ranges self.ruby_system = ruby_system self.directory = RubyDirectoryMemory() # Connect this directory to the memory side. self.memory = mem_ctrls.port self.connectQueues(ruby_system) def connectQueues(self, ruby_system): self.requestFromCache = MessageBuffer(ordered = True) self.requestFromCache.slave = ruby_system.network.master self.responseFromCache = MessageBuffer(ordered = True) self.responseFromCache.slave = ruby_system.network.master self.responseToCache = MessageBuffer(ordered = True) self.responseToCache.master = ruby_system.network.slave self.forwardToCache = MessageBuffer(ordered = True) self.forwardToCache.master = ruby_system.network.slave self.responseFromMemory = MessageBuffer()
Now, we can implement the Ruby system object. For this object, the
constructor is simple. It just checks the SCons variable
be sure that we are using the right configuration file for the protocol
that was compiled. We cannot create the controllers in the constructor
because they require a pointer to the this object. If we were to create
them in the constructor, there would be a circular dependence in the
SimObject hierarchy which will cause infinite recursion in when the
system in instantiated with
class MyCacheSystem(RubySystem): def __init__(self): if buildEnv['PROTOCOL'] != 'MSI': fatal("This system assumes MSI from learning gem5!") super(MyCacheSystem, self).__init__()
Instead of create the controllers in the constructor, we create a new
function to create all of the needed objects:
setup. First, we create
the network. We will look at this object next. With the network, we need
to set the number of virtual networks in the system.
Next, we instantiate all of the controllers. Here, we use a single global list of the controllers to make it easier to connect them to the network later. However, for more complicated cache topologies, it can make sense to use multiple lists of controllers. We create one L1 cache for each CPU and one directory for the system.
Then, we instantiate all of the sequencers, one for each CPU. Each sequencer needs a pointer to the instruction and data cache to simulate the correct latency when initially accessing the cache. In more complicated systems, you also have to create sequencers for other objects like DMA controllers.
After creating the sequencers, we set the sequencer variable on each L1 cache controller.
Then, we connect all of the controllers to the network and call the
setup_buffers function on the network.
We then have to set the “port proxy” for both the Ruby system and the
system for making functional accesses (e.g., loading the binary in SE
Finally, we connect all of the CPUs to the ruby system. In this example, we assume that there are only CPU sequencers so the first CPU is connected to the first sequencer, and so on. We also have to connect the TLBs and interrupt ports (if we are using x86).
def setup(self, system, cpus, mem_ctrls): self.network = MyNetwork(self) self.number_of_virtual_networks = 3 self.network.number_of_virtual_networks = 3 self.controllers = \ [L1Cache(system, self, cpu) for cpu in cpus] + \ [DirController(self, system.mem_ranges, mem_ctrls)] self.sequencers = [RubySequencer(version = i, # I/D cache is combined and grab from ctrl icache = self.controllers[i].cacheMemory, dcache = self.controllers[i].cacheMemory, clk_domain = self.controllers[i].clk_domain, ) for i in range(len(cpus))] for i,c in enumerate(self.controllers[0:len(self.sequencers)]): c.sequencer = self.sequencers[i] self.num_of_sequencers = len(self.sequencers) self.network.connectControllers(self.controllers) self.network.setup_buffers() self.sys_port_proxy = RubyPortProxy() system.system_port = self.sys_port_proxy.slave for i,cpu in enumerate(cpus): cpu.icache_port = self.sequencers[i].slave cpu.dcache_port = self.sequencers[i].slave isa = buildEnv['TARGET_ISA'] if isa == 'x86': cpu.interrupts.pio = self.sequencers[i].master cpu.interrupts.int_master = self.sequencers[i].slave cpu.interrupts.int_slave = self.sequencers[i].master if isa == 'x86' or isa == 'arm': cpu.itb.walker.port = self.sequencers[i].slave cpu.dtb.walker.port = self.sequencers[i].slave
Finally, the last object we have to implement is the network. The
constructor is simple, but we need to declare an empty list for the list
of network interfaces (
Most of the code is in
connectControllers. This function implements a
very simple, unrealistic point-to-point network. In other words, every
controller has a direct link to every other controller.
The Ruby network is made of three parts: routers that route data from one router to another or to external controllers, external links that link a controller to a router, and internal links that link two routers together. First, we create a router for each controller. Then, we create an external link from that router to the controller. Finally, we add all of the “internal” links. Each router is connected to all other routers to make the point-to-point network.
class MyNetwork(SimpleNetwork): def __init__(self, ruby_system): super(MyNetwork, self).__init__() self.netifs =  self.ruby_system = ruby_system def connectControllers(self, controllers): self.routers = [Switch(router_id = i) for i in range(len(controllers))] self.ext_links = [SimpleExtLink(link_id=i, ext_node=c, int_node=self.routers[i]) for i, c in enumerate(controllers)] link_count = 0 self.int_links =  for ri in self.routers: for rj in self.routers: if ri == rj: continue # Don't connect a router to itself! link_count += 1 self.int_links.append(SimpleIntLink(link_id = link_count, src_node = ri, dst_node = rj))
You can download the complete