| Message ID | cone.1350828020.51066.30326.1000@monster.email-scan.com |
|---|---|
| State | New |
| Headers | show |
On 10/21/2012 04:00 PM, Sam Varshavchik wrote: > Based on a casual browsing of clock_gettime(3), CLOCK_MONOTONIC_RAW > seems to be a better fit for std::chrono::steady_clock's requirements > as given in 20.11.7.2, with recent Linux kernels, > > Something like this: Please always CC library patches to libstdc++@gcc.gnu.org. > > > Index: libstdc++-v3/src/c++11/chrono.cc > =================================================================== > --- libstdc++-v3/src/c++11/chrono.cc (revision 192652) > +++ libstdc++-v3/src/c++11/chrono.cc (working copy) > @@ -70,7 +70,11 @@ > { > timespec tp; > // -EINVAL, -EFAULT > +#ifdef CLOCK_MONOTONIC_RAW > + clock_gettime(CLOCK_MONOTONIC_RAW, &tp); > +#else > clock_gettime(CLOCK_MONOTONIC, &tp); > +#endif > return time_point(duration(chrono::seconds(tp.tv_sec) > + chrono::nanoseconds(tp.tv_nsec))); > } >
* Sam Varshavchik: > Based on a casual browsing of clock_gettime(3), CLOCK_MONOTONIC_RAW > seems to be a better fit for std::chrono::steady_clock's requirements > as given in 20.11.7.2, with recent Linux kernels, Are the Linux clock semantics documented somewhere in detail? > +#ifdef CLOCK_MONOTONIC_RAW > + clock_gettime(CLOCK_MONOTONIC_RAW, &tp); > +#else > clock_gettime(CLOCK_MONOTONIC, &tp); > +#endif If a #define is available at compile time, it's not necessarily the case that the feature is present at run time. If CLOCK_MONOTONIC_RAW is indeed what we want, we need fallback code.
On Sun, Oct 21, 2012 at 12:11 PM, Paolo Carlini <paolo.carlini@oracle.com> wrote: \>> @@ -70,7 +70,11 @@ >> { >> timespec tp; >> // -EINVAL, -EFAULT >> +#ifdef CLOCK_MONOTONIC_RAW >> + clock_gettime(CLOCK_MONOTONIC_RAW, &tp); >> +#else >> clock_gettime(CLOCK_MONOTONIC, &tp); >> +#endif >> return time_point(duration(chrono::seconds(tp.tv_sec) >> + chrono::nanoseconds(tp.tv_nsec))); >> } That'll have to be something like #ifdef CLOCK_MONOTONIC_RAW if (clock_gettime(CLOCK_MONOTONIC_RAW, &tp) != 0) #endif clock_gettime(CLOCK_MONOTONIC, &tp); Only way out of this is when you introduce a check for a minimum kernel ABI somewhere, just like glibc does.
Florian Weimer writes: > * Sam Varshavchik: > > > Based on a casual browsing of clock_gettime(3), CLOCK_MONOTONIC_RAW > > seems to be a better fit for std::chrono::steady_clock's requirements > > as given in 20.11.7.2, with recent Linux kernels, > > Are the Linux clock semantics documented somewhere in detail? clock_gettime(3) says: # CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific) # Similar to CLOCK_MONOTONIC, but provides access to a raw hard‐ # ware-based time that is not subject to NTP adjustments. That looks like a better fit for what 20.11.7.2 says steady_clock should be.
Index: libstdc++-v3/src/c++11/chrono.cc =================================================================== --- libstdc++-v3/src/c++11/chrono.cc (revision 192652) +++ libstdc++-v3/src/c++11/chrono.cc (working copy) @@ -70,7 +70,11 @@ { timespec tp; // -EINVAL, -EFAULT +#ifdef CLOCK_MONOTONIC_RAW + clock_gettime(CLOCK_MONOTONIC_RAW, &tp); +#else clock_gettime(CLOCK_MONOTONIC, &tp); +#endif return time_point(duration(chrono::seconds(tp.tv_sec) + chrono::nanoseconds(tp.tv_nsec))); }