Write a C program which measures the speed of a context switch on a UNIX/Linux system.Context
The switch itself is one-way, so the best we can do in userland (I assume that's what you are asking) is to measure sort of an RTT, from our process to another and back. The other process also takes time to do its work. We can of course make two or more processes cooperate on this, but the thing is that the kernel doesn't guarantee that one of our processes is going to be picked next.
// Compile with g++ latencybench.cc -o latencybench -lboost_thread-mt
// Should also work on MSVC and other platforms supported by Boost.
#include <boost/format.hpp>
#include <boost/thread/thread.hpp>
#include <boost/date_time.hpp>
#include <algorithm>
#include <cstdlib>
#include <csignal>
volatile bool m_quit = false;
extern "C" void sighandler(int) {
m_quit = true;
}
std::string num(unsigned val) {
if (val == 1) return "one occurrence";
return boost::lexical_cast<std::string>(val) + " occurrences";
}
int main(int argc, char** argv) {
using namespace boost::posix_time;
std::signal(SIGINT, sighandler);
std::signal(SIGTERM, sighandler);
time_duration duration = milliseconds(10);
if (argc > 1) {
try {
if (argc != 2) throw 1;
unsigned ms = boost::lexical_cast<unsigned>(argv[1]);
if (ms > 1000) throw 2;
duration = milliseconds(ms);
} catch (...) {
std::cerr << "Usage: " << argv[0] << " milliseconds" << std::endl;
return EXIT_FAILURE;
}
}
typedef std::map<long, unsigned> Durations;
Durations durations;
unsigned samples = 0, wrongsamples = 0;
unsigned max = 0;
long last = -1;
std::cout << "Measuring actual sleep delays when requesting " << duration.total_milliseconds() << " ms: (Ctrl+C when done)" << std::endl;
ptime begin = boost::get_system_time();
while (!m_quit) {
ptime start = boost::get_system_time();
boost::this_thread::sleep(start + duration);
long actual = (boost::get_system_time() - start).total_milliseconds();
++samples;
unsigned num = ++durations[actual];
if (actual != last) {
std::cout << "\r " << actual << " ms " << std::flush;
last = actual;
}
if (actual != duration.total_milliseconds()) {
++wrongsamples;
if (num > max) max = num;
std::cout << "spike at " << start - begin << std::endl;
last = -1;
}
}
if (samples == 0) return 0;
std::cout << "\rTotal measurement duration: " << boost::get_system_time() - begin << "\n";
std::cout << "Number of samples collected: " << samples << "\n";
std::cout << "Incorrect delay count: " << wrongsamples << boost::format(" (%.2f %%)") % (100.0 * wrongsamples / samples) << "\n\n";
std::cout << "Histogram of actual delays:\n\n";
unsigned correctsamples = samples - wrongsamples;
const unsigned line = 60;
double scale = 1.0;
char ch = '+';
if (max > line) {
scale = double(line) / max;
ch = '*';
}
double correctscale = 1.0;
if (correctsamples > line) correctscale = double(line) / correctsamples;
for (Durations::const_iterator it = durations.begin(); it != durations.end(); ++it) {
std::string bar;
if (it->first == duration.total_milliseconds()) bar = std::string(correctscale * it->second, '>');
else bar = std::string(scale * it->second, ch);
std::cout << boost::format("%5d ms | %s %d") % it->first % bar % it->second << std::endl;
}
std::cout << "\n";
std::string indent(30, ' ');
std::cout << indent << "+-- Legend ----------------------------------\n";
std::cout << indent << "| > " << num(1.0 / correctscale) << " (of " << duration.total_milliseconds() << " ms delay)\n";
if (wrongsamples > 0) std::cout << indent << "| " << ch << " " << num(1.0 / scale) << " (of any other delay)\n";
}
You are given a the source to a application which is crashing when run.