Userspace Filesystems Framework for NetBSD
The quality of timekeeping is critical for many network protocols and measurement tools. TSC-I2 (TSC-Internet2) ensures accuracy and precision by making TSC rate calibration a continuous process, thus the accuracy of interpolation parameters could be ensured, which in turn results in satisfying clock precision. TSC-I2 maintains a soft clock of its own, periodically comparing this clock to the system clock. During each comparison, it synchronizes itself with the system clock, and adjusts the interpolation rate based on the offset and rate errors regarding to system clock. Whenever the accuracy of the soft clock is ensured, TSC-I2 uses this clock to report time to the library user; otherwise, the system clock value is reported. The advantage of this design is that system clock is enhanced rather than substituted.
The clock discipline algorithm is enlightened by NTP. A state-machine-controlled PLL (Phase Lock Loop) traps the rate-induced phase difference between TSC clock and system clock. Rate wander is captured within one loop delay, and corrected in three to four following loops. To avoid incorrect recognition of noise as a rate-induced error, two filters—a popcorn filter and a spike detector—are used. There are two usage modes: DAEMON and CLIENT. In DAEMON mode, a standalone daemon takes charge of timekeeping, serving one or more clients. In CLIENT mode, the library creates a thread running within the hosting process. Thus, it minimizes the application's external dependency. There are also clear distinctions between TSC-I2 algorithms and its NTP counterparts, mainly due to the different natures of referencing sources. Readers interested in TSC-I2 internals can visit the project web site, where more details are illustrated.
TSC-I2, which is fully implemented in around 2000 lines of C, is fairly lightweight. It has been published under the Open Source License at http://tsc-xluo.sourceforge.net/. TSC-I2 currently supports IA32, AMD64, and Power PC architectures, as well as Linux, FreeBSD, Mac OS X, and Microsoft Windows.
DDJ
