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Update README to mention UseRealTime for wallclock time measurements.

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Also adding a use case in the API header.

Fixes #170
This commit is contained in:
Eli Bendersky 2015-12-30 06:01:19 -08:00
parent f662e8be5b
commit c7ab1b987b
2 changed files with 19 additions and 8 deletions
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17
README.md
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@ -145,9 +145,10 @@ Three macros are provided for adding benchmark templates.
#define BENCHMARK_TEMPLATE2(func, arg1, arg2)
```
In a multithreaded test, it is guaranteed that none of the threads will start
until all have called KeepRunning, and all will have finished before KeepRunning
returns false. As such, any global setup or teardown you want to do can be
In a multithreaded test (benchmark invoked by multiple threads simultaneously),
it is guaranteed that none of the threads will start until all have called
KeepRunning, and all will have finished before KeepRunning returns false. As
such, any global setup or teardown you want to do can be
wrapped in a check against the thread index:
```c++
@ -165,6 +166,16 @@ static void BM_MultiThreaded(benchmark::State& state) {
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:
```c++
BENCHMARK(BM_test)->Range(8, 8<<10)->UseRealTime();
```
Without `UseRealTime`, CPU time is used by default.
To prevent a value or expression from being optimized away by the compiler
the `benchmark::DoNotOptimize(...)` function can be used.

10
include/benchmark/benchmark_api.h
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@ -417,11 +417,11 @@ public:
// option overrides the `benchmark_min_time` flag.
Benchmark* MinTime(double t);
// If a particular benchmark is I/O bound, or if for some reason CPU
// timings are not representative, call this method. If called, the elapsed
// time will be used to control how many iterations are run, and in the
// printing of items/second or MB/seconds values. If not called, the cpu
// time used by the benchmark will be used.
// If a particular benchmark is I/O bound, runs multiple threads internally or
// if for some reason CPU timings are not representative, call this method. If
// called, the elapsed time will be used to control how many iterations are
// run, and in the printing of items/second or MB/seconds values. If not
// called, the cpu time used by the benchmark will be used.
Benchmark* UseRealTime();
// Support for running multiple copies of the same benchmark concurrently