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PMSI: Predictable MSI cache coherence protocol

This is the gem5 code repository for Predictable MSI (PMSI) presented in our RTAS'17 paper: Predictable Cache Coherence for Multi-core Real-time Systems.

Getting started

  • The Gem5 simulator has been modified to support predictable snoopy bus architecture, and trace based simulation.
  • $GEM5 refers to the top level directory where gem5 resides.
  • File $GEM5/build_opts/PMSI contains information for building the PMSI system, and $GEM5/configs/ruby/ provides information about the component (caches, network, DRAM) connections in the system. Note that these files do not need to be changed unless the cache hierarchy is extended.
  • To build PMSI, execute scons $GEM5/build/PMSI/gem5.opt -j8. The -j8 is to accelerate the build process.

Memory trace simulation

  • Gem5 has been extended to run memory trace based execution.
  • The trace based simulation injects traces into the ruby memory model thereby bypassing the core/processor.
  • Trace based simulation can be enabled by setting the TRACE flag in src/cpu/testers/rubytest/Trace.hh.
  • The trace based simulation reads from a file trace.trc that consists of lines of requests of the form Addr OP time. OP is of type RD for read and WR for write, and time is a positive integer value that denotes the arrival time of the memory request to the memory hierarchy.

Running PMSI

  • To run PMSI, the following command line options need to be used for correct functioning: --ruby, --cpu-type=timing, --topology=Crossbar --cpu-clock=xGHz --ruby-clock=yGHz.
  • Other options such as cache sizes, memory size, cache associativity, and number of cores can be configured accordingly.
  • For binary/normal execution, supply the binary name and the associated arguments using the -c and -o flags respectively. For example, to run FFT of the Splash2 benchmark: ./build/X86_MSI_Snooping_One_Level_RT/gem5.opt --stats-file=FFT-pmsi-stats.out ./configs/example/ -I 50000000 --ruby -n4 --cpu-type=timing --l1d_size=16kB --l1i_size=16kB --l1d_assoc=1 --l1i_assoc=1 --mem-size=4194304kB --topology=Crossbar --cpu-clock=2GHz --ruby-clock=2GHz --mem-type=SimpleMemory -c $SPLASH/splash2/codes/kernels/fft/fft -o "-m 10 -p 4 -t"
  • For trace based execution, populate the trace.trc file with the necessary memory operations and execute: ./build/X86_MSI_Snooping_One_Level_RT/gem5.opt ./configs/example/ --ruby-clock=2GHz --ruby --cpu-clock=2GHz --topology=Crossbar --mem-type=SimpleMemory -n 4 --mem-size=4194304kB --wakeup_freq=1.

Predictable bus arbitration

  • We extend Gem5 to support a snoopy bus with a predictable arbiter. The predictable aribiter operates in a time division multiplexed (TDM) manner.
  • The NPROC and SLOT_WIDTH parameters in src/cpu/testers/rubyTest/Trace.hh control the number of requestors and the TDM slot width per requestor.


  • Feel free to contact us for questions regarding PMSI.