Nice priority of a Linux thread.

nice.cc

#include <iostream> // cout, cerr, endl
#include <memory> // shared_ptr
#include <sys/time.h> // setpriority
#include <sys/resource.h> // setpriority

/**
 * Set low nice priority in the current Linux thread.\n
 * Usage: \code auto&& restorePriority = ioNice(); \endcode.
 */
static shared_ptr<void> ioNice (int priority = 19) {
  shared_ptr<void> restorePriority;
#ifdef __linux__
  // Lower the I/O priority of the thread (cf. `man ionice`).
  const auto tid = gettid();
  std::cout << "Setting *nice* priority of TID " << tid << " to " << priority << "." << std::endl;
  errno = 0; const auto oldNicePriority = ::getpriority (PRIO_PROCESS, tid);
  if (oldNicePriority == -1 && errno) std::cerr << "getpriority: " << ::strerror (errno) << std::endl;
  if (::setpriority (PRIO_PROCESS, tid, priority)) std::cerr << "setpriority: " << ::strerror (errno) << std::endl;

  restorePriority.reset ((void*) nullptr, [=](void*) {
    std::cout << "Restoring *nice* priority of TID " << tid << " to " << oldNicePriority << "." << std::endl;
    if (::setpriority (PRIO_PROCESS, tid, oldNicePriority)) std::cerr << "setpriority: " << ::strerror (errno) << std::endl;
  });
#endif
  return restorePriority;
};