Understanding gem5 statistics and output

In addition to any information which your simulation script prints out, after running gem5, there are three files generated in a directory called m5out:

config.ini
Contains a list of every SimObject created for the simulation and the values for its parameters.
config.json
The same as config.ini, but in json format.
stats.txt
A text representation of all of the gem5 statistics registered for the simulation.

Where these files are created can be controlled by

--outdir=DIR, -d DIR

The directory to be created which hold the gem5 output files, including config.ini, config.json, stats.txt, and possibly other. The files in this directory are overwritten if they already exist.

config.ini

This file is the definitive version of what was simulated. All of the parameters for each SimObject that is simulated, whether they were set in the configuration scripts or the defaults were used, are shown in this file.

Below is pulled from the config.ini generated when the simple.py configuration file from Creating a simple configuration script is run.

[root]
type=Root
children=system
eventq_index=0
full_system=false
sim_quantum=0
time_sync_enable=false
time_sync_period=100000000000
time_sync_spin_threshold=100000000

[system]
type=System
children=clk_domain cpu dvfs_handler mem_ctrl membus
boot_osflags=a
cache_line_size=64
clk_domain=system.clk_domain
eventq_index=0
init_param=0
kernel=
kernel_addr_check=true
load_addr_mask=1099511627775
load_offset=0
mem_mode=timing
mem_ranges=0:536870911
memories=system.mem_ctrl

[system.clk_domain]
type=SrcClockDomain
children=voltage_domain
clock=1000
domain_id=-1
eventq_index=0
init_perf_level=0
voltage_domain=system.clk_domain.voltage_domain

...

[system.membus]
type=CoherentXBar
clk_domain=system.clk_domain
eventq_index=0
header_cycles=1
snoop_filter=Null
system=system
use_default_range=false
width=8
master=system.cpu.interrupts[0].pio system.cpu.interrupts[0].int_slave system.mem_ctrl.port
slave=system.cpu.icache_port system.cpu.dcache_port system.cpu.interrupts[0].int_master system.system_port

Here we see that at the beginning of the description of each SimObject is first it’s name as created in the configuration file surrounded by square brackets (e.g., [system.membus]).

Next, every parameter of the SimObject is shown with it’s value, including parameters not explicitly set in the configuration file. For instance, the configuration file sets the clock domain to be 1 GHz (1000 ticks in this case). However, it did not set the cache line size (which is 64 in the system) object.

The config.ini file is a valuable tool for ensuring that you are simulating what you think you’re simulating. There are many possible ways to set default values, and to override default values, in gem5. It is a “best-practice” to always check the config.ini as a sanity check that values set in the configuration file are propagated to the actual SimObject instantiation.

stats.txt

gem5 has a flexible statistics generating system. gem5 statistics is covered in some detail on the gem5 wiki site. Each instantiation of a SimObject has it’s own statistics. At the end of simulation, or when special statistic-dumping commands are issued, the current state of the statistics for all SimObjects is dumped to a file.

First, the statistics file contains general statistics about the execution:

---------- Begin Simulation Statistics ----------
sim_seconds                                  0.000346                       # Number of seconds simulated
sim_ticks                                   345518000                       # Number of ticks simulated
final_tick                                  345518000                       # Number of ticks from beginning of simulation (restored from checkpoints and never reset)
sim_freq                                 1000000000000                       # Frequency of simulated ticks
host_inst_rate                                 144400                       # Simulator instruction rate (inst/s)
host_op_rate                                   260550                       # Simulator op (including micro ops) rate (op/s)
host_tick_rate                             8718625183                       # Simulator tick rate (ticks/s)
host_mem_usage                                 778640                       # Number of bytes of host memory used
host_seconds                                     0.04                       # Real time elapsed on the host
sim_insts                                        5712                       # Number of instructions simulated
sim_ops                                         10314                       # Number of ops (including micro ops) simulated

The statistic dump begins with ---------- Begin Simulation Statistics ----------. There may be multiple of these in a single file if there are multiple statistic dumps during the gem5 execution. This is common for long running applications, or when restoring from checkpoints.

Each statistic has a name (first column), a value (second column), and a description (last column preceded by #).

Most of the statistics are self explanatory from their descriptions. A couple of important statistics are sim_seconds which is the total simulated time for the simulation, sim_insts which is the number of instructions committed by the CPU, and host_inst_rate which tells you the performance of gem5.

Next, the SimObjects’ statistics are printed. For instance, the memory controller statistics. This has information like the bytes read by each component and the average bandwidth used by those components.

system.mem_ctrl.bytes_read::cpu.inst            58264                       # Number of bytes read from this memory
system.mem_ctrl.bytes_read::cpu.data             7167                       # Number of bytes read from this memory
system.mem_ctrl.bytes_read::total               65431                       # Number of bytes read from this memory
system.mem_ctrl.bytes_inst_read::cpu.inst        58264                       # Number of instructions bytes read from this memory
system.mem_ctrl.bytes_inst_read::total          58264                       # Number of instructions bytes read from this memory
system.mem_ctrl.bytes_written::cpu.data          7160                       # Number of bytes written to this memory
system.mem_ctrl.bytes_written::total             7160                       # Number of bytes written to this memory
system.mem_ctrl.num_reads::cpu.inst              7283                       # Number of read requests responded to by this memory
system.mem_ctrl.num_reads::cpu.data              1084                       # Number of read requests responded to by this memory
system.mem_ctrl.num_reads::total                 8367                       # Number of read requests responded to by this memory
system.mem_ctrl.num_writes::cpu.data              941                       # Number of write requests responded to by this memory
system.mem_ctrl.num_writes::total                 941                       # Number of write requests responded to by this memory
system.mem_ctrl.bw_read::cpu.inst           168627973                       # Total read bandwidth from this memory (bytes/s)
system.mem_ctrl.bw_read::cpu.data            20742769                       # Total read bandwidth from this memory (bytes/s)
system.mem_ctrl.bw_read::total              189370742                       # Total read bandwidth from this memory (bytes/s)
system.mem_ctrl.bw_inst_read::cpu.inst      168627973                       # Instruction read bandwidth from this memory (bytes/s)
system.mem_ctrl.bw_inst_read::total         168627973                       # Instruction read bandwidth from this memory (bytes/s)
system.mem_ctrl.bw_write::cpu.data           20722509                       # Write bandwidth from this memory (bytes/s)
system.mem_ctrl.bw_write::total              20722509                       # Write bandwidth from this memory (bytes/s)
system.mem_ctrl.bw_total::cpu.inst          168627973                       # Total bandwidth to/from this memory (bytes/s)
system.mem_ctrl.bw_total::cpu.data           41465278                       # Total bandwidth to/from this memory (bytes/s)
system.mem_ctrl.bw_total::total             210093251                       # Total bandwidth to/from this memory (bytes/s)

Later in the file is the CPU statistics, which contains information on the number of syscalls, the number of branches, total committed instructions, etc.

system.cpu.apic_clk_domain.clock                16000                       # Clock period in ticks
system.cpu.workload.num_syscalls                   11                       # Number of system calls
system.cpu.numCycles                           345518                       # number of cpu cycles simulated
system.cpu.numWorkItemsStarted                      0                       # number of work items this cpu started
system.cpu.numWorkItemsCompleted                    0                       # number of work items this cpu completed
system.cpu.committedInsts                        5712                       # Number of instructions committed
system.cpu.committedOps                         10314                       # Number of ops (including micro ops) committed
system.cpu.num_int_alu_accesses                 10205                       # Number of integer alu accesses
system.cpu.num_fp_alu_accesses                      0                       # Number of float alu accesses
system.cpu.num_func_calls                         221                       # number of times a function call or return occured
system.cpu.num_conditional_control_insts          986                       # number of instructions that are conditional controls
system.cpu.num_int_insts                        10205                       # number of integer instructions
system.cpu.num_fp_insts                             0                       # number of float instructions
system.cpu.num_int_register_reads               19296                       # number of times the integer registers were read
system.cpu.num_int_register_writes               7977                       # number of times the integer registers were written
system.cpu.num_fp_register_reads                    0                       # number of times the floating registers were read
system.cpu.num_fp_register_writes                   0                       # number of times the floating registers were written
system.cpu.num_cc_register_reads                 7020                       # number of times the CC registers were read
system.cpu.num_cc_register_writes                3825                       # number of times the CC registers were written
system.cpu.num_mem_refs                          2025                       # number of memory refs
system.cpu.num_load_insts                        1084                       # Number of load instructions
system.cpu.num_store_insts                        941                       # Number of store instructions
system.cpu.num_idle_cycles                   0.001000                       # Number of idle cycles
system.cpu.num_busy_cycles               345517.999000                       # Number of busy cycles
system.cpu.not_idle_fraction                 1.000000                       # Percentage of non-idle cycles
system.cpu.idle_fraction                     0.000000                       # Percentage of idle cycles
system.cpu.Branches                              1306                       # Number of branches fetched