AutoCounter: Profiling with Out-of-Band Performance Counter Collection¶
FireSim can provide visibility into a simulated CPU’s architectural and microarchitectural state over the course of execution through the use of counters. These are similar to performance counters provided by processor vendors, and more general counters provided by architectural simulators.
This functionality is provided by the AutoCounter feature (introduced in our FirePerf paper at ASPLOS 2020), and can be used for profiling and debugging. Since AutoCounter injects counters only in simulation (unlike target-level performance counters), these counters do not affect the behavior of the simulated machine, no matter how often they are sampled.
Ad-hoc Performance Counters¶
AutoCounter enables the addition of ad-hoc counters using the PerfCounter
function. The PerfCounter
function takes 3 arguments: A boolean signal to
be counted, a counter label, and the counter description. Here is an example
counter declaration:
midas.targetutils.PerfCounter(s1_pc, "s1_pc", "stage 1 program counter")
Building a Design with AutoCounter¶
To enable AutoCounter when building a design, prepend the WithAutoCounter
config to your PLATFORM_CONFIG
. During compilation, FireSim will print the
signals it is generating counters for. If AutoCounter has been enabled, the
autocounter_t
bridge driver will also be automatically instantiated.
Rocket Chip Cover Functions¶
The cover function is applied to various signals in the Rocket Chip generator repository to mark points of interest (i.e., interesting signals) in the RTL. Tools are free to provide their own implementation of this function to process these signals as they wish. In FireSim, these functions can be used as a hook for automatic generation of counters.
Since cover functions are embedded throughout the code of Rocket Chip (and possibly other code repositories), AutoCounter provides a filtering mechanism based on module granularity. As such, only cover functions that appear within selected modules will generate counters.
The filtered modules can be indicated using one of two methods:
- A module selection annotation within the top-level configuration
implementation. To use this method, add the
AutoCounterCoverModuleAnnotation
annotation with the name of the module for which you want the cover functions to be turned into AutoCounters. The following example will generate counters from cover functions within theStreamWriter
module:
class FireSimDUT(implicit p: Parameters) extends Subsystem
with HasHierarchicalBusTopology
with CanHaveMasterAXI4MemPort
with HasPeripheryBootROM
with HasPeripherySerial
with HasPeripheryUART
with HasPeripheryIceNIC
with HasPeripheryBlockDevice
with HasTraceIO
{
override lazy val module = new FireSimModuleImp(this)
chisel3.experimental.annotate(AutoCounterCoverModuleAnnotation("StreamWriter"))
}
- An input file with a list of module names. This input file is named
autocounter-covermodules.txt
, and includes a list of module names separated by new lines (no commas).
AutoCounter Runtime Parameters¶
AutoCounter currently takes a single runtime configurable parameter, defined
under the [autocounter]
section in the config_runtime.ini
file. The
readrate
parameter defines the rate at which the counters should be read,
and is measured in target-cycles. Hence, if the read-rate is defined to be 100,
the simulator will read and print the values of the counters every 100 cycles.
By default, the read-rate is set to 0 cycles, which is equivalent to disabling
AutoCounter.
[autocounter]
# read counters every 100 cycles
readrate=100
Upon setting this value, when you run a workload, an AutoCounter output file
will be placed in the sim_slot_<slot #>
directory on the F1 instance under
the name AUTOCOUNTERFILE0
.
Note
AutoCounter is designed as a coarse-grained observability mechanism. It assumes the counters will be read at intervals greater than O(10000) cycles. If you intend on reading counters at a finer granularity, please consider using synthesizable printfs (otherwise, simulation performance may degrade more than necessary)
Using TracerV Trigger with AutoCounter¶
In order to collect AutoCounter results from only from a particular region of interest in the simulation, AutoCounter has been integrated with TracerV triggers. See the Setting a TracerV Trigger section for more information.
AutoCounter using Synthesizable Printfs¶
The AutoCounter transformation in the Golden Gate compiler includes a legacy
mode that uses Synthesizable Printfs (learn more about these on the
Printf Synthesis: Capturing RTL printf Calls when Running on the FPGA page) to export counter results rather than
a dedicated Bridge. This mode can be enabled by prepending the
WithAutoCounterCoverPrintf
config to your PLATFORM_CONFIG
instead of
WithAutoCounterCover
. In this mode, the counter values will be printed
using a synthesizable printf every time the counter is incremented (hence, you
will observe a series of printfs incrementing by 1). Nevertheless, the printf
statements include the exact cycle of the printf, and therefore this mode may
be useful for fine grained observation of counters. The counter values will be
printed to the same output stream as other synthesizable printfs. This mode
may export a large amount of data (since it prints every cycle a counter
increments), and therefore it is not recommended unless you require such high
fidelity.