... > Who would have thought that rdtsc() in applications can be a problem. > Interfaces to query time exist for a reason and it's documented by > Microsoft: > > https://learn.microsoft.com/en-us/windows/win32/dxtecharts/game-timing-and-multicore-processors > > But sure, reading documentation is overrated... That eve says: "Multiprocessor and dual-core systems do not guarantee synchronization of their cycle counters between cores." . > Synchronizing TSC by writing the TSC MSR is fragile as hell. This has > been tried so often and never reliably passed all synchronization tests > on a wide range of systems. > > It kinda works on single socket, but not on larger systems. > > We spent an insane amount of time to make timekeeping correct and I'm > not interested at all to deal with the fallout of such a mechanim. I've wondered whether the TSC ought to be deliberately mis-synchronised? So the high order bits are effectively the cpu number. It has to be said that using it as a time source was fundamentally a bad idea. Sometimes (eg micro benchmarks) you really want a TSC. You can extract one from the performance counters, but it is hard, root only, and the library functions have high and variable overhead. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
