gem5/src/cpu/testers/gpu_ruby_test/README

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This directory contains a tester for gem5 GPU protocols. Unlike the Ruby random
teter, this tester does not rely on sequential consistency. Instead, it
assumes tested protocols supports release consistency.

----- Getting Started -----

To start using the tester quickly, you can use the following example command
line to get running immediately:

build/GCN3_X86/gem5.opt configs/example/ruby_gpu_random_test.py \
            --test-length=1000 --system-size=medium --cache-size=small

An overview of the main command line options is as follows. For all options
use `build/GCN3_X86/gem5.opt configs/example/ruby_gpu_random_test.py --help`
or see the configuration file.

 * --cache-size (small, large): Use smaller sizes for testing evict, etc.
 * --system-size (small, medium, large): Effectively the number of threads in
                 the GPU model. Large size will have more contention. Larger
                 sizes are useful for checking contention.
 * --episode-length (short, medium, long): Number of loads and stores in an
                 episode. Episodes will also have atomics mixed in. See below
                 for a definition of episode.
 * --test-length (int): Number of episodes to execute. This will determine the
                 amount of time the tester runs for. Longer time will stress
                 the protocol harder.

The remainder of this file describes the theory behind the tester design and
a link to a more detailed research paper is provided at the end.

----- Theory Overview -----

The GPU Ruby tester creates a system consisting of both CPU threads and GPU
wavefronts. CPU threads are scalar, so there is one lane per CPU thread. GPU
wavefront may have multiple lanes. The number of lanes is initialized when
a thread/wavefront is created.

Each thread/wavefront executes a number of episodes. Each episode is a series
of memory actions (i.e., atomic, load, store, acquire and release). In a
wavefront, all lanes execute the same sequence of actions, but they may target
different addresses. One can think of an episode as a critical section which
is bounded by a lock acquire in the beginning and a lock release at the end. An
episode consists of actions in the following order:

1 - Atomic action
2 - Acquire action
3 - A number of load and store actions
4 - Release action
5 - Atomic action that targets the same address as (1) does

There are two separate set of addresses: atomic and non-atomic. Atomic actions
target only atomic addresses. Load and store actions target only non-atomic
addresses. Memory addresses are all 4-byte aligned in the tester.

To test false sharing cases in which both atomic and non-atomic addresses are
placed in the same cache line, we abstract out the concept of memory addresses
from the tester's perspective by introducing the concept of location. Locations
are numbered from 0 to N-1 (if there are N addresses). The first X locations
[0..X-1] are atomic locations, and the rest are non-atomic locations.
The 1-1 mapping between locations and addresses are randomly created when the
tester is initialized.

Per load and store action, its target location is selected so that there is no
data race in the generated stream of memory requests at any time during the
test. Since in Data-Race-Free model, the memory system's behavior is undefined
in data race cases, we exclude data race scenarios from our protocol test.

Once location per load/store action is determined, each thread/wavefront either
loads current value at the location or stores an incremental value to that
location. The tester maintains a table tracking all last writers and their
written values, so we know what value should be returned from a load and what
value should be written next at a particular location. Value returned from a
load must match with the value written by the last writer.

----- Directory Structure -----

ProtocolTester.hh/cc -- This is the main tester class that orchestrates the
                        entire test.
AddressManager.hh/cc -- This manages address space, randomly maps address to
                        location, generates locations for all episodes,
                        maintains per-location last writer and validates
                        values returned from load actions.
TesterThread.hh/cc   -- This is abstract class for CPU threads and GPU
                        wavefronts. It generates and executes a series of
                        episodes.
CpuThread.hh/cc      -- Thread class for CPU threads. Not fully implemented yet
GpuWavefront.hh/cc   -- Thread class for GPU wavefronts.
Episode.hh/cc        -- Class to encapsulate an episode, notably including
                        episode load/store structure and ordering.

For more detail, please see the following paper:

T. Ta, X. Zhang, A. Gutierrez and B. M. Beckmann, "Autonomous Data-Race-Free
GPU Testing," 2019 IEEE International Symposium on Workload Characterization
(IISWC), Orlando, FL, USA, 2019, pp. 81-92, doi:
10.1109/IISWC47752.2019.9042019.