Use H/W Monotonic clock and updates to AE (#7644)

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.

1 files changed, 12 insertions, 0 deletions

diff --git a/README.md b/README.md
index d8f8f7880..46b89c392 100644
--- a/README.md
+++ b/README.md
@@ -106,6 +106,18 @@ To compile against jemalloc on Mac OS X systems, use:
 
     % make MALLOC=jemalloc
 
+Monotonic clock
+---------------
+
+By default, Redis will build using the POSIX clock_gettime function as the
+monotonic clock source.  On most modern systems, the internal processor clock
+can be used to improve performance.  Cautions can be found here: 
+    http://oliveryang.net/2015/09/pitfalls-of-TSC-usage/
+
+To build with support for the processor's internal instruction clock, use:
+
+    % make CFLAGS="-DUSE_PROCESSOR_CLOCK"
+
 Verbose build
 -------------