On Wed, Aug 11, 2021 at 05:13:45PM +0200, JP wrote: > I understand that performance is much better when using CLOCK_REALTIME (as > opposed to working with a PHC directly) because a PHC generally exists > behind a bus on external hardware. Am I correct in assuming this wouldn't > be an issue with a PHC vclock?
A virtual PHC is nothing more than a Linux kernel timecounter/cyclecounter structure on top of a physical PHC which the kernel keeps completely free-running. The timecounter/cyclecounter structure provides 'virtual' time bases on top of the same physical counter, i.e. when you adjust the frequency or the absolute time of the virtual clock, only a software correction factor is modified, which is applied to all raw clock values read from the physical PHC. So the clock_gettime() returned by a virtual PHC is a simple "a * x + b" translation of the clock_gettime() returned by the backing PHC. This is also what allows you to have many virtual clocks on top of the same physical one. So to answer your question: the performance of reading the time from a vclock is exactly the same as the performance of doing that from the physical PHC that's backing it. If the access to the physical PHC is slow, the access to its vclocks is equally so, which makes timerfds and so on not very friendly. _______________________________________________ Linuxptp-users mailing list Linuxptp-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linuxptp-users
