Memory Simulator (mem-sim)
                    ==========================
                                 
Author: Vivek Seshadri
Affiliation: Carnegie Mellon University

1. Brief Overview
-----------------

mem-sim is a fast and flexible memory subsystem simulator. The
goal of the simulator was to enable ease of simulating many
different memory system configurations (primarily the cache). The
simulator only models the back-end and can be attached to any
front-end CPU model. In this code base, we have a simple
trace-based front-end Out-of-Order CPU model.

Please cite the following work if you use this simulator as part
of your evaluation in your research/development. This simulator is
the basis of evaluation in the following works.

Vivek Seshadri, Onur Mutlu, Michael A. Kozuch, and Todd C. Mowry,
"The Evicted-Address Filter: A Unified Mechanism to Address Both
Cache Pollution and Thrashing," Proceedings of the 21st ACM
International Conference on Parallel Architectures and Compilation
Techniques (PACT), Minneapolis, MN, September 2012.


2. Simulator Model
------------------

The simulator models a memory subsystem as a collection of
components. For example, in a three-level cache hierarchy, the L1
cache of a core is a component; the shared L3 cache is another
component. The configuration is then described as a connection
between these components. Each core in the system has an input
component --- the component that talks directly to the core. When
the front-end generates a memory request, it is sent to the input
component. Each component has two operations: 1) processing a
forward request from the CPU and 2) processing the return request
from the downstream components.

When a request is received from the CPU side, the first operation
is invoked. This operation can either mark the request as
serviced. If yes, the request is sent back to the CPU or the
previous component. If not, the request in sent to the next
component in the hierarchy. If there is no next component, the
request is marked as served by default.

Once a request is marked as served, it is sent to the previous
component by invoking the second operation of the previous
component.

Both the forward and the return operation can optionally add a
latency to each request. The accumulated return latency to the
core is used to determine how long the CPU stalls.

For a cache component, the forward operation is a cache lookup
that determines if the request is a hit or miss and the return
operation is a cache fill that inserts the block into the cache.

The following figure shows a memory system with two cores and five
components.

      Core 1 ---> L1-1 ---> L2-1
                                \
                                |---> L3
                                /
      Core 2 ---> L1-2 ---> L2-2


Each core has it's own private L1 and L2 cache and the both the L2
caches are connected to a shared L3 cache. Since the L3 cache has
no next component, all requests to the L3 cache will be marked as
served --- modeling a perfect L3.

Each core has a specific path in this memory subsystem graph. This
path is used to determine the next/previous components for each
request, based on the core that generated the request.


3. Compiling and Running the Simulator
--------------------------------------

To compile the simulator, simply run make from the Simulator
folder.

mem-sim/Simulator> make

To run the simulator, we need two files: a simulator definition
file within the folder "mem-sim/Simulator/Definitions" (say
def-file) and a configuration (describing the parameter file for
each component in the simulator) inside the folder
"mem-sim/Simulator/Configs/def-file" (say conf-file).

To run the simulator, simply run the run_simulator.py script from
the scripts folder.

mem-sim> ./Simulator/Scripts/run_simulator.py --definition
def-file --configuration conf-file  --folder <result-folder>
--workload <workload-name> <other-simulation-parameters>

You can run multiple configurations simultaneously by using wild
cards. The results are stored in the folder
"<result-folder>/def-file/conf-file/<workload-name>". Results can
be parsed using the parse_results.py file in the Scripts
directory.