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Jim Brunner authored
Update adds a general source for retrieving a monotonic time. In addition, AE has been updated to utilize the new monotonic clock for timer processing. This performance improvement is **not** enabled in a default build due to various H/W compatibility concerns, see README.md for details. It does however change the default use of gettimeofday with clock_gettime and somewhat improves performance. This update provides the following 1. An interface for retrieving a monotonic clock. getMonotonicUs returns a uint64_t (aka monotime) with the number of micro-seconds from an arbitrary point. No more messing with tv_sec/tv_usec. Simple routines are provided for measuring elapsed milli-seconds or elapsed micro-seconds (the most common use case for a monotonic timer). No worries about time moving backwards. 2. High-speed assembler implementation for x86 and ARM. The standard method for retrieving the monotonic clock is POSIX.1b (1993): clock_gettime(CLOCK_MONOTONIC, timespec*). However, most modern processors provide a constant speed instruction clock which can be retrieved in a fraction of the time that it takes to call clock_gettime. For x86, this is provided by the RDTSC instruction. For ARM, this is provided by the CNTVCT_EL0 instruction. As a compile-time option, these high-speed timers can be chosen. (Default is POSIX clock_gettime.) 3. Refactor of event loop timers. The timer processing in ae.c has been refactored to use the new monotonic clock interface. This results in simpler/cleaner logic and improved performance.
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