benchmark
A library to support the benchmarking of functions, similar to unit-tests.
Discussion group: https://groups.google.com/d/forum/benchmark-discuss
IRC channel: https://freenode.net #googlebenchmark
Known issues and common problems
Additional Tooling Documentation
Assembly Testing Documentation
Building
The basic steps for configuring and building the library look like this:
$ git clone https://github.com/google/benchmark.git
# Benchmark requires Google Test as a dependency. Add the source tree as a subdirectory.
$ git clone https://github.com/google/googletest.git benchmark/googletest
$ mkdir build && cd build
$ cmake -G <generator> [options] ../benchmark
# Assuming a makefile generator was used
$ make
Note that Google Benchmark requires Google Test to build and run the tests. This dependency can be provided two ways:
- Checkout the Google Test sources into
benchmark/googletestas above. - Otherwise, if
-DBENCHMARK_DOWNLOAD_DEPENDENCIES=ONis specified during configuration, the library will automatically download and build any required dependencies.
If you do not wish to build and run the tests, add -DBENCHMARK_ENABLE_GTEST_TESTS=OFF
to CMAKE_ARGS.
Installation Guide
For Ubuntu and Debian Based System
First make sure you have git and cmake installed (If not please install it)
sudo apt-get install git
sudo apt-get install cmake
Now, let's clone the repository and build it
git clone https://github.com/google/benchmark.git
cd benchmark
git clone https://github.com/google/googletest.git
mkdir build
cd build
cmake .. -DCMAKE_BUILD_TYPE=RELEASE
make
We need to install the library globally now
sudo make install
Now you have google/benchmark installed in your machine Note: Don't forget to link to pthread library while building
Stable and Experimental Library Versions
The main branch contains the latest stable version of the benchmarking library; the API of which can be considered largely stable, with source breaking changes being made only upon the release of a new major version.
Newer, experimental, features are implemented and tested on the
v2 branch. Users who wish
to use, test, and provide feedback on the new features are encouraged to try
this branch. However, this branch provides no stability guarantees and reserves
the right to change and break the API at any time.
##Prerequisite knowledge
Before attempting to understand this framework one should ideally have some familiarity with the structure and format of the Google Test framework, upon which it is based. Documentation for Google Test, including a "Getting Started" (primer) guide, is available here: https://github.com/google/googletest/blob/master/googletest/docs/Documentation.md
Example usage
Basic usage
Define a function that executes the code to be measured.
#include <benchmark/benchmark.h>
static void BM_StringCreation(benchmark::State& state) {
for (auto _ : state)
std::string empty_string;
}
// Register the function as a benchmark
BENCHMARK(BM_StringCreation);
// Define another benchmark
static void BM_StringCopy(benchmark::State& state) {
std::string x = "hello";
for (auto _ : state)
std::string copy(x);
}
BENCHMARK(BM_StringCopy);
BENCHMARK_MAIN();
Don't forget to inform your linker to add benchmark library e.g. through
-lbenchmark compilation flag. Alternatively, you may leave out the
BENCHMARK_MAIN(); at the end of the source file and link against
-lbenchmark_main to get the same default behavior.
The benchmark library will reporting the timing for the code within the for(...) loop.
Passing arguments
Sometimes a family of benchmarks can be implemented with just one routine that
takes an extra argument to specify which one of the family of benchmarks to
run. For example, the following code defines a family of benchmarks for
measuring the speed of memcpy() calls of different lengths:
static void BM_memcpy(benchmark::State& state) {
char* src = new char[state.range(0)];
char* dst = new char[state.range(0)];
memset(src, 'x', state.range(0));
for (auto _ : state)
memcpy(dst, src, state.range(0));
state.SetBytesProcessed(int64_t(state.iterations()) *
int64_t(state.range(0)));
delete[] src;
delete[] dst;
}
BENCHMARK(BM_memcpy)->Arg(8)->Arg(64)->Arg(512)->Arg(1<<10)->Arg(8<<10);
The preceding code is quite repetitive, and can be replaced with the following short-hand. The following invocation will pick a few appropriate arguments in the specified range and will generate a benchmark for each such argument.
BENCHMARK(BM_memcpy)->Range(8, 8<<10);
By default the arguments in the range are generated in multiples of eight and the command above selects [ 8, 64, 512, 4k, 8k ]. In the following code the range multiplier is changed to multiples of two.
BENCHMARK(BM_memcpy)->RangeMultiplier(2)->Range(8, 8<<10);
Now arguments generated are [ 8, 16, 32, 64, 128, 256, 512, 1024, 2k, 4k, 8k ].
You might have a benchmark that depends on two or more inputs. For example, the following code defines a family of benchmarks for measuring the speed of set insertion.
static void BM_SetInsert(benchmark::State& state) {
std::set<int> data;
for (auto _ : state) {
state.PauseTiming();
data = ConstructRandomSet(state.range(0));
state.ResumeTiming();
for (int j = 0; j < state.range(1); ++j)
data.insert(RandomNumber());
}
}
BENCHMARK(BM_SetInsert)
->Args({1<<10, 128})
->Args({2<<10, 128})
->Args({4<<10, 128})
->Args({8<<10, 128})
->Args({1<<10, 512})
->Args({2<<10, 512})
->Args({4<<10, 512})
->Args({8<<10, 512});
The preceding code is quite repetitive, and can be replaced with the following short-hand. The following macro will pick a few appropriate arguments in the product of the two specified ranges and will generate a benchmark for each such pair.
BENCHMARK(BM_SetInsert)->Ranges({{1<<10, 8<<10}, {128, 512}});
For more complex patterns of inputs, passing a custom function to Apply allows
programmatic specification of an arbitrary set of arguments on which to run the
benchmark. The following example enumerates a dense range on one parameter,
and a sparse range on the second.
static void CustomArguments(benchmark::internal::Benchmark* b) {
for (int i = 0; i <= 10; ++i)
for (int j = 32; j <= 1024*1024; j *= 8)
b->Args({i, j});
}
BENCHMARK(BM_SetInsert)->Apply(CustomArguments);
Calculate asymptotic complexity (Big O)
Asymptotic complexity might be calculated for a family of benchmarks. The following code will calculate the coefficient for the high-order term in the running time and the normalized root-mean square error of string comparison.
static void BM_StringCompare(benchmark::State& state) {
std::string s1(state.range(0), '-');
std::string s2(state.range(0), '-');
for (auto _ : state) {
benchmark::DoNotOptimize(s1.compare(s2));
}
state.SetComplexityN(state.range(0));
}
BENCHMARK(BM_StringCompare)
->RangeMultiplier(2)->Range(1<<10, 1<<18)->Complexity(benchmark::oN);
As shown in the following invocation, asymptotic complexity might also be calculated automatically.
BENCHMARK(BM_StringCompare)
->RangeMultiplier(2)->Range(1<<10, 1<<18)->Complexity();
The following code will specify asymptotic complexity with a lambda function, that might be used to customize high-order term calculation.
BENCHMARK(BM_StringCompare)->RangeMultiplier(2)
->Range(1<<10, 1<<18)->Complexity([](int n)->double{return n; });
Templated benchmarks
Templated benchmarks work the same way: This example produces and consumes
messages of size sizeof(v) range_x times. It also outputs throughput in the
absence of multiprogramming.
template <class Q> int BM_Sequential(benchmark::State& state) {
Q q;
typename Q::value_type v;
for (auto _ : state) {
for (int i = state.range(0); i--; )
q.push(v);
for (int e = state.range(0); e--; )
q.Wait(&v);
}
// actually messages, not bytes:
state.SetBytesProcessed(
static_cast<int64_t>(state.iterations())*state.range(0));
}
BENCHMARK_TEMPLATE(BM_Sequential, WaitQueue<int>)->Range(1<<0, 1<<10);
Three macros are provided for adding benchmark templates.
#ifdef BENCHMARK_HAS_CXX11
#define BENCHMARK_TEMPLATE(func, ...) // Takes any number of parameters.
#else // C++ < C++11
#define BENCHMARK_TEMPLATE(func, arg1)
#endif
#define BENCHMARK_TEMPLATE1(func, arg1)
#define BENCHMARK_TEMPLATE2(func, arg1, arg2)
A Faster KeepRunning loop
In C++11 mode, a ranged-based for loop should be used in preference to
the KeepRunning loop for running the benchmarks. For example:
static void BM_Fast(benchmark::State &state) {
for (auto _ : state) {
FastOperation();
}
}
BENCHMARK(BM_Fast);
The reason the ranged-for loop is faster than using KeepRunning, is
because KeepRunning requires a memory load and store of the iteration count
ever iteration, whereas the ranged-for variant is able to keep the iteration count
in a register.
For example, an empty inner loop of using the ranged-based for method looks like:
# Loop Init
mov rbx, qword ptr [r14 + 104]
call benchmark::State::StartKeepRunning()
test rbx, rbx
je .LoopEnd
.LoopHeader: # =>This Inner Loop Header: Depth=1
add rbx, -1
jne .LoopHeader
.LoopEnd:
Compared to an empty KeepRunning loop, which looks like:
.LoopHeader: # in Loop: Header=BB0_3 Depth=1
cmp byte ptr [rbx], 1
jne .LoopInit
.LoopBody: # =>This Inner Loop Header: Depth=1
mov rax, qword ptr [rbx + 8]
lea rcx, [rax + 1]
mov qword ptr [rbx + 8], rcx
cmp rax, qword ptr [rbx + 104]
jb .LoopHeader
jmp .LoopEnd
.LoopInit:
mov rdi, rbx
call benchmark::State::StartKeepRunning()
jmp .LoopBody
.LoopEnd:
Unless C++03 compatibility is required, the ranged-for variant of writing the benchmark loop should be preferred.
Passing arbitrary arguments to a benchmark
In C++11 it is possible to define a benchmark that takes an arbitrary number
of extra arguments. The BENCHMARK_CAPTURE(func, test_case_name, ...args)
macro creates a benchmark that invokes func with the benchmark::State as
the first argument followed by the specified args....
The test_case_name is appended to the name of the benchmark and
should describe the values passed.
template <class ...ExtraArgs>
void BM_takes_args(benchmark::State& state, ExtraArgs&&... extra_args) {
[...]
}
// Registers a benchmark named "BM_takes_args/int_string_test" that passes
// the specified values to `extra_args`.
BENCHMARK_CAPTURE(BM_takes_args, int_string_test, 42, std::string("abc"));
Note that elements of ...args may refer to global variables. Users should
avoid modifying global state inside of a benchmark.
Using RegisterBenchmark(name, fn, args...)
The RegisterBenchmark(name, func, args...) function provides an alternative
way to create and register benchmarks.
RegisterBenchmark(name, func, args...) creates, registers, and returns a
pointer to a new benchmark with the specified name that invokes
func(st, args...) where st is a benchmark::State object.
Unlike the BENCHMARK registration macros, which can only be used at the global
scope, the RegisterBenchmark can be called anywhere. This allows for
benchmark tests to be registered programmatically.
Additionally RegisterBenchmark allows any callable object to be registered
as a benchmark. Including capturing lambdas and function objects.
For Example:
auto BM_test = [](benchmark::State& st, auto Inputs) { /* ... */ };
int main(int argc, char** argv) {
for (auto& test_input : { /* ... */ })
benchmark::RegisterBenchmark(test_input.name(), BM_test, test_input);
benchmark::Initialize(&argc, argv);
benchmark::RunSpecifiedBenchmarks();
}
Multithreaded benchmarks
In a multithreaded test (benchmark invoked by multiple threads simultaneously),
it is guaranteed that none of the threads will start until all have reached
the start of the benchmark loop, and all will have finished before any thread
exits the benchmark loop. (This behavior is also provided by the KeepRunning()
API) As such, any global setup or teardown can be wrapped in a check against the thread
index:
static void BM_MultiThreaded(benchmark::State& state) {
if (state.thread_index == 0) {
// Setup code here.
}
for (auto _ : state) {
// Run the test as normal.
}
if (state.thread_index == 0) {
// Teardown code here.
}
}
BENCHMARK(BM_MultiThreaded)->Threads(2);
If the benchmarked code itself uses threads and you want to compare it to single-threaded code, you may want to use real-time ("wallclock") measurements for latency comparisons:
BENCHMARK(BM_test)->Range(8, 8<<10)->UseRealTime();
Without UseRealTime, CPU time is used by default.
Manual timing
For benchmarking something for which neither CPU time nor real-time are
correct or accurate enough, completely manual timing is supported using
the UseManualTime function.
When UseManualTime is used, the benchmarked code must call
SetIterationTime once per iteration of the benchmark loop to
report the manually measured time.
An example use case for this is benchmarking GPU execution (e.g. OpenCL
or CUDA kernels, OpenGL or Vulkan or Direct3D draw calls), which cannot
be accurately measured using CPU time or real-time. Instead, they can be
measured accurately using a dedicated API, and these measurement results
can be reported back with SetIterationTime.
static void BM_ManualTiming(benchmark::State& state) {
int microseconds = state.range(0);
std::chrono::duration<double, std::micro> sleep_duration {
static_cast<double>(microseconds)
};
for (auto _ : state) {
auto start = std::chrono::high_resolution_clock::now();
// Simulate some useful workload with a sleep
std::this_thread::sleep_for(sleep_duration);
auto end = std::chrono::high_resolution_clock::now();
auto elapsed_seconds =
std::chrono::duration_cast<std::chrono::duration<double>>(
end - start);
state.SetIterationTime(elapsed_seconds.count());
}
}
BENCHMARK(BM_ManualTiming)->Range(1, 1<<17)->UseManualTime();
Preventing optimisation
To prevent a value or expression from being optimized away by the compiler
the benchmark::DoNotOptimize(...) and benchmark::ClobberMemory()
functions can be used.
static void BM_test(benchmark::State& state) {
for (auto _ : state) {
int x = 0;
for (int i=0; i < 64; ++i) {
benchmark::DoNotOptimize(x += i);
}
}
}
DoNotOptimize(<expr>) forces the result of <expr> to be stored in either
memory or a register. For GNU based compilers it acts as read/write barrier
for global memory. More specifically it forces the compiler to flush pending
writes to memory and reload any other values as necessary.
Note that DoNotOptimize(<expr>) does not prevent optimizations on <expr>
in any way. <expr> may even be removed entirely when the result is already
known. For example:
/* Example 1: `<expr>` is removed entirely. */
int foo(int x) { return x + 42; }
while (...) DoNotOptimize(foo(0)); // Optimized to DoNotOptimize(42);
/* Example 2: Result of '<expr>' is only reused */
int bar(int) __attribute__((const));
while (...) DoNotOptimize(bar(0)); // Optimized to:
// int __result__ = bar(0);
// while (...) DoNotOptimize(__result__);
The second tool for preventing optimizations is ClobberMemory(). In essence
ClobberMemory() forces the compiler to perform all pending writes to global
memory. Memory managed by block scope objects must be "escaped" using
DoNotOptimize(...) before it can be clobbered. In the below example
ClobberMemory() prevents the call to v.push_back(42) from being optimized
away.
static void BM_vector_push_back(benchmark::State& state) {
for (auto _ : state) {
std::vector<int> v;
v.reserve(1);
benchmark::DoNotOptimize(v.data()); // Allow v.data() to be clobbered.
v.push_back(42);
benchmark::ClobberMemory(); // Force 42 to be written to memory.
}
}
Note that ClobberMemory() is only available for GNU or MSVC based compilers.
Set time unit manually
If a benchmark runs a few milliseconds it may be hard to visually compare the measured times, since the output data is given in nanoseconds per default. In order to manually set the time unit, you can specify it manually:
BENCHMARK(BM_test)->Unit(benchmark::kMillisecond);
Controlling number of iterations
In all cases, the number of iterations for which the benchmark is run is
governed by the amount of time the benchmark takes. Concretely, the number of
iterations is at least one, not more than 1e9, until CPU time is greater than
the minimum time, or the wallclock time is 5x minimum time. The minimum time is
set as a flag --benchmark_min_time or per benchmark by calling MinTime on
the registered benchmark object.
Reporting the mean, median and standard deviation by repeated benchmarks
By default each benchmark is run once and that single result is reported. However benchmarks are often noisy and a single result may not be representative of the overall behavior. For this reason it's possible to repeatedly rerun the benchmark.
The number of runs of each benchmark is specified globally by the
--benchmark_repetitions flag or on a per benchmark basis by calling
Repetitions on the registered benchmark object. When a benchmark is run more
than once the mean, median and standard deviation of the runs will be reported.
Additionally the --benchmark_report_aggregates_only={true|false} flag or
ReportAggregatesOnly(bool) function can be used to change how repeated tests
are reported. By default the result of each repeated run is reported. When this
option is true only the mean, median and standard deviation of the runs is reported.
Calling ReportAggregatesOnly(bool) on a registered benchmark object overrides
the value of the flag for that benchmark.
User-defined statistics for repeated benchmarks
While having mean, median and standard deviation is nice, this may not be enough for everyone. For example you may want to know what is the largest observation, e.g. because you have some real-time constraints. This is easy. The following code will specify a custom statistic to be calculated, defined by a lambda function.
void BM_spin_empty(benchmark::State& state) {
for (auto _ : state) {
for (int x = 0; x < state.range(0); ++x) {
benchmark::DoNotOptimize(x);
}
}
}
BENCHMARK(BM_spin_empty)
->ComputeStatistics("max", [](const std::vector<double>& v) -> double {
return *(std::max_element(std::begin(v), std::end(v)));
})
->Arg(512);
Fixtures
Fixture tests are created by
first defining a type that derives from ::benchmark::Fixture and then
creating/registering the tests using the following macros:
BENCHMARK_F(ClassName, Method)BENCHMARK_DEFINE_F(ClassName, Method)BENCHMARK_REGISTER_F(ClassName, Method)
For Example:
class MyFixture : public benchmark::Fixture {};
BENCHMARK_F(MyFixture, FooTest)(benchmark::State& st) {
for (auto _ : st) {
...
}
}
BENCHMARK_DEFINE_F(MyFixture, BarTest)(benchmark::State& st) {
for (auto _ : st) {
...
}
}
/* BarTest is NOT registered */
BENCHMARK_REGISTER_F(MyFixture, BarTest)->Threads(2);
/* BarTest is now registered */
Templated fixtures
Also you can create templated fixture by using the following macros:
BENCHMARK_TEMPLATE_F(ClassName, Method, ...)BENCHMARK_TEMPLATE_DEFINE_F(ClassName, Method, ...)
For example:
template<typename T>
class MyFixture : public benchmark::Fixture {};
BENCHMARK_TEMPLATE_F(MyFixture, IntTest, int)(benchmark::State& st) {
for (auto _ : st) {
...
}
}
BENCHMARK_TEMPLATE_DEFINE_F(MyFixture, DoubleTest, double)(benchmark::State& st) {
for (auto _ : st) {
...
}
}
BENCHMARK_REGISTER_F(MyFixture, DoubleTest)->Threads(2);
User-defined counters
You can add your own counters with user-defined names. The example below will add columns "Foo", "Bar" and "Baz" in its output:
static void UserCountersExample1(benchmark::State& state) {
double numFoos = 0, numBars = 0, numBazs = 0;
for (auto _ : state) {
// ... count Foo,Bar,Baz events
}
state.counters["Foo"] = numFoos;
state.counters["Bar"] = numBars;
state.counters["Baz"] = numBazs;
}
The state.counters object is a std::map with std::string keys
and Counter values. The latter is a double-like class, via an implicit
conversion to double&. Thus you can use all of the standard arithmetic
assignment operators (=,+=,-=,*=,/=) to change the value of each counter.
In multithreaded benchmarks, each counter is set on the calling thread only. When the benchmark finishes, the counters from each thread will be summed; the resulting sum is the value which will be shown for the benchmark.
The Counter constructor accepts two parameters: the value as a double
and a bit flag which allows you to show counters as rates and/or as
per-thread averages:
// sets a simple counter
state.counters["Foo"] = numFoos;
// Set the counter as a rate. It will be presented divided
// by the duration of the benchmark.
state.counters["FooRate"] = Counter(numFoos, benchmark::Counter::kIsRate);
// Set the counter as a thread-average quantity. It will
// be presented divided by the number of threads.
state.counters["FooAvg"] = Counter(numFoos, benchmark::Counter::kAvgThreads);
// There's also a combined flag:
state.counters["FooAvgRate"] = Counter(numFoos,benchmark::Counter::kAvgThreadsRate);
When you're compiling in C++11 mode or later you can use insert() with
std::initializer_list:
// With C++11, this can be done:
state.counters.insert({{"Foo", numFoos}, {"Bar", numBars}, {"Baz", numBazs}});
// ... instead of:
state.counters["Foo"] = numFoos;
state.counters["Bar"] = numBars;
state.counters["Baz"] = numBazs;
Counter reporting
When using the console reporter, by default, user counters are are printed at
the end after the table, the same way as bytes_processed and
items_processed. This is best for cases in which there are few counters,
or where there are only a couple of lines per benchmark. Here's an example of
the default output:
------------------------------------------------------------------------------
Benchmark Time CPU Iterations UserCounters...
------------------------------------------------------------------------------
BM_UserCounter/threads:8 2248 ns 10277 ns 68808 Bar=16 Bat=40 Baz=24 Foo=8
BM_UserCounter/threads:1 9797 ns 9788 ns 71523 Bar=2 Bat=5 Baz=3 Foo=1024m
BM_UserCounter/threads:2 4924 ns 9842 ns 71036 Bar=4 Bat=10 Baz=6 Foo=2
BM_UserCounter/threads:4 2589 ns 10284 ns 68012 Bar=8 Bat=20 Baz=12 Foo=4
BM_UserCounter/threads:8 2212 ns 10287 ns 68040 Bar=16 Bat=40 Baz=24 Foo=8
BM_UserCounter/threads:16 1782 ns 10278 ns 68144 Bar=32 Bat=80 Baz=48 Foo=16
BM_UserCounter/threads:32 1291 ns 10296 ns 68256 Bar=64 Bat=160 Baz=96 Foo=32
BM_UserCounter/threads:4 2615 ns 10307 ns 68040 Bar=8 Bat=20 Baz=12 Foo=4
BM_Factorial 26 ns 26 ns 26608979 40320
BM_Factorial/real_time 26 ns 26 ns 26587936 40320
BM_CalculatePiRange/1 16 ns 16 ns 45704255 0
BM_CalculatePiRange/8 73 ns 73 ns 9520927 3.28374
BM_CalculatePiRange/64 609 ns 609 ns 1140647 3.15746
BM_CalculatePiRange/512 4900 ns 4901 ns 142696 3.14355
If this doesn't suit you, you can print each counter as a table column by
passing the flag --benchmark_counters_tabular=true to the benchmark
application. This is best for cases in which there are a lot of counters, or
a lot of lines per individual benchmark. Note that this will trigger a
reprinting of the table header any time the counter set changes between
individual benchmarks. Here's an example of corresponding output when
--benchmark_counters_tabular=true is passed:
---------------------------------------------------------------------------------------
Benchmark Time CPU Iterations Bar Bat Baz Foo
---------------------------------------------------------------------------------------
BM_UserCounter/threads:8 2198 ns 9953 ns 70688 16 40 24 8
BM_UserCounter/threads:1 9504 ns 9504 ns 73787 2 5 3 1
BM_UserCounter/threads:2 4775 ns 9550 ns 72606 4 10 6 2
BM_UserCounter/threads:4 2508 ns 9951 ns 70332 8 20 12 4
BM_UserCounter/threads:8 2055 ns 9933 ns 70344 16 40 24 8
BM_UserCounter/threads:16 1610 ns 9946 ns 70720 32 80 48 16
BM_UserCounter/threads:32 1192 ns 9948 ns 70496 64 160 96 32
BM_UserCounter/threads:4 2506 ns 9949 ns 70332 8 20 12 4
--------------------------------------------------------------
Benchmark Time CPU Iterations
--------------------------------------------------------------
BM_Factorial 26 ns 26 ns 26392245 40320
BM_Factorial/real_time 26 ns 26 ns 26494107 40320
BM_CalculatePiRange/1 15 ns 15 ns 45571597 0
BM_CalculatePiRange/8 74 ns 74 ns 9450212 3.28374
BM_CalculatePiRange/64 595 ns 595 ns 1173901 3.15746
BM_CalculatePiRange/512 4752 ns 4752 ns 147380 3.14355
BM_CalculatePiRange/4k 37970 ns 37972 ns 18453 3.14184
BM_CalculatePiRange/32k 303733 ns 303744 ns 2305 3.14162
BM_CalculatePiRange/256k 2434095 ns 2434186 ns 288 3.1416
BM_CalculatePiRange/1024k 9721140 ns 9721413 ns 71 3.14159
BM_CalculatePi/threads:8 2255 ns 9943 ns 70936
Note above the additional header printed when the benchmark changes from
BM_UserCounter to BM_Factorial. This is because BM_Factorial does
not have the same counter set as BM_UserCounter.
Exiting Benchmarks in Error
When errors caused by external influences, such as file I/O and network
communication, occur within a benchmark the
State::SkipWithError(const char* msg) function can be used to skip that run
of benchmark and report the error. Note that only future iterations of the
KeepRunning() are skipped. For the ranged-for version of the benchmark loop
Users must explicitly exit the loop, otherwise all iterations will be performed.
Users may explicitly return to exit the benchmark immediately.
The SkipWithError(...) function may be used at any point within the benchmark,
including before and after the benchmark loop.
For example:
static void BM_test(benchmark::State& state) {
auto resource = GetResource();
if (!resource.good()) {
state.SkipWithError("Resource is not good!");
// KeepRunning() loop will not be entered.
}
for (state.KeepRunning()) {
auto data = resource.read_data();
if (!resource.good()) {
state.SkipWithError("Failed to read data!");
break; // Needed to skip the rest of the iteration.
}
do_stuff(data);
}
}
static void BM_test_ranged_fo(benchmark::State & state) {
state.SkipWithError("test will not be entered");
for (auto _ : state) {
state.SkipWithError("Failed!");
break; // REQUIRED to prevent all further iterations.
}
}
Running a subset of the benchmarks
The --benchmark_filter=<regex> option can be used to only run the benchmarks
which match the specified <regex>. For example:
$ ./run_benchmarks.x --benchmark_filter=BM_memcpy/32
Run on (1 X 2300 MHz CPU )
2016-06-25 19:34:24
Benchmark Time CPU Iterations
----------------------------------------------------
BM_memcpy/32 11 ns 11 ns 79545455
BM_memcpy/32k 2181 ns 2185 ns 324074
BM_memcpy/32 12 ns 12 ns 54687500
BM_memcpy/32k 1834 ns 1837 ns 357143
Output Formats
The library supports multiple output formats. Use the
--benchmark_format=<console|json|csv> flag to set the format type. console
is the default format.
The Console format is intended to be a human readable format. By default the format generates color output. Context is output on stderr and the tabular data on stdout. Example tabular output looks like:
Benchmark Time(ns) CPU(ns) Iterations
----------------------------------------------------------------------
BM_SetInsert/1024/1 28928 29349 23853 133.097kB/s 33.2742k items/s
BM_SetInsert/1024/8 32065 32913 21375 949.487kB/s 237.372k items/s
BM_SetInsert/1024/10 33157 33648 21431 1.13369MB/s 290.225k items/s
The JSON format outputs human readable json split into two top level attributes.
The context attribute contains information about the run in general, including
information about the CPU and the date.
The benchmarks attribute contains a list of every benchmark run. Example json
output looks like:
{
"context": {
"date": "2015/03/17-18:40:25",
"num_cpus": 40,
"mhz_per_cpu": 2801,
"cpu_scaling_enabled": false,
"build_type": "debug"
},
"benchmarks": [
{
"name": "BM_SetInsert/1024/1",
"iterations": 94877,
"real_time": 29275,
"cpu_time": 29836,
"bytes_per_second": 134066,
"items_per_second": 33516
},
{
"name": "BM_SetInsert/1024/8",
"iterations": 21609,
"real_time": 32317,
"cpu_time": 32429,
"bytes_per_second": 986770,
"items_per_second": 246693
},
{
"name": "BM_SetInsert/1024/10",
"iterations": 21393,
"real_time": 32724,
"cpu_time": 33355,
"bytes_per_second": 1199226,
"items_per_second": 299807
}
]
}
The CSV format outputs comma-separated values. The context is output on stderr
and the CSV itself on stdout. Example CSV output looks like:
name,iterations,real_time,cpu_time,bytes_per_second,items_per_second,label
"BM_SetInsert/1024/1",65465,17890.7,8407.45,475768,118942,
"BM_SetInsert/1024/8",116606,18810.1,9766.64,3.27646e+06,819115,
"BM_SetInsert/1024/10",106365,17238.4,8421.53,4.74973e+06,1.18743e+06,
Output Files
The library supports writing the output of the benchmark to a file specified
by --benchmark_out=<filename>. The format of the output can be specified
using --benchmark_out_format={json|console|csv}. Specifying
--benchmark_out does not suppress the console output.
Debug vs Release
By default, benchmark builds as a debug library. You will see a warning in the output when this is the case. To build it as a release library instead, use:
cmake -DCMAKE_BUILD_TYPE=Release
To enable link-time optimisation, use
cmake -DCMAKE_BUILD_TYPE=Release -DBENCHMARK_ENABLE_LTO=true
If you are using gcc, you might need to set GCC_AR and GCC_RANLIB cmake cache variables, if autodetection fails.
If you are using clang, you may need to set LLVMAR_EXECUTABLE, LLVMNM_EXECUTABLE and LLVMRANLIB_EXECUTABLE cmake cache variables.
Linking against the library
When the library is built using GCC it is necessary to link with -pthread,
due to how GCC implements std::thread.
For GCC 4.x failing to link to pthreads will lead to runtime exceptions, not linker errors. See issue #67 for more details.
Compiler Support
Google Benchmark uses C++11 when building the library. As such we require a modern C++ toolchain, both compiler and standard library.
The following minimum versions are strongly recommended build the library:
- GCC 4.8
- Clang 3.4
- Visual Studio 2013
- Intel 2015 Update 1
Anything older may work.
Note: Using the library and its headers in C++03 is supported. C++11 is only required to build the library.
Disable CPU frequency scaling
If you see this error:
***WARNING*** CPU scaling is enabled, the benchmark real time measurements may be noisy and will incur extra overhead.
you might want to disable the CPU frequency scaling while running the benchmark:
sudo cpupower frequency-set --governor performance
./mybench
sudo cpupower frequency-set --governor powersave
Known Issues
Windows with CMake
- Users must manually link
shlwapi.lib. Failure to do so may result in unresolved symbols.
Solaris
- Users must explicitly link with kstat library (-lkstat compilation flag).