Communications of the ACM

Another nuance is that these two values together describe the translation between a CPU’s TSC and the system’s gethrtime(), and they are estimates. That means two important things: They need to be updated regularly to correct for error in the calibration and estimation; and they need to be set and read atomically. This is where the cmpxchg16b instruction enters.

Additionally, this calibration work is performed every five seconds in a separate thread, and we attempt to make that thread high priority on a real-time scheduler. It turns out that this all works quite well, even without the ability to change priority or scheduling class.

Gravy

Since we’re clearly having to correct for skew to align with the system gethrtime(), and the point in the past to which gethrtime() is relative is arbitrary (according to the documentation), we’ve elected to make that “arbitrary” point the Unix epoch. No additional instructions are required, and now the replacement gethrtime() can be used to power gettimeofday(). Therefore, y = mx + b is actually implemented as:

nano _ second _ since _ epoch =

nanos _ per _ tick mtev _ rdtscp() + skew

Obviously, we’ll pick up changes to the wall clock (via adjtime() et al.) only when we recalibrate.

Safety

Obviously, things can and do go wrong. A variety of fail-safe mechanisms are in place to ensure proper behavior when the optimizations become unsafe. By default, if the lack of an invariant TSC is detected, the system is disabled. If a calibration loop fails for too long ( 15 seconds), the CPU is marked as bad and disabled. During rudimentary performance tests, if the system’s gethrtime() can beat the emulation, then we disable. If all those tests pass, we still check to see if the underlying system can perform gettimeofday() faster than we can emulate it; if so, we disable gettimeofday() emulation. The goal is for mtev_gethrtime() to be as fast as or faster than gethrtime() and for mtev_gettimeof-day() to be as fast as or faster than gettimeofday().

Results

The overall results are better than ex-
pected. The original goal was simply to
provide a way for our implementation
on Illumos to meet the performance
of Linux. The value of ZFS is deeply
profound, and while Linux has some
performance advantages in specific
arenas, that doesn’t matter much if
you have undetectable corruption of
the data you store.

Further optimization is possible in the implementation, but we’ve stopped for now, having achieved the initial goals. Additionally, for the purposes of this test, we have built the code portably. We can find a couple of nanoseconds if we compile -march=native on machines supporting the AVX (Advanced Vector Extensions) instruction set.

It is true that an approximately 40ns gethrtime() can be considered slow enough, relative to microsecond-level efforts, that very prudent selection is still necessary. It is also true that 40ns gethrtime() can open up a new world of possibilities for user-space instrumentation. It has certainly opened our eyes to some startling things.

Acknowledgment

This all comes for free with https:// github.com/circonus-labs/libmtev/ blob/master/src/utils/ mtev_time.com (see https://github.com/circonus-labs/ libmtev/blob/master/src/utils/mtev_ time.c for reference).

As of this writing, Linux and Illumos are supported platforms, and Darwin and FreeBSD do not have “faster time” support. The faster time support in libmtev was a collaborative effort between Riley Berton and Theo Schlossnagle.

Related articles on queue.acm.org

Passively Measuring TCP Round-Trip Times Stephen D. Strowes http://queue.acm.org/detail.cfm?id=2539132 The One-Second War (What Time Will You Die?)

Poul-Henning Kamp http://queue.acm.org/detail.cfm?id=1967009

Principles of Robust Timing over the Internet

Julien Ridoux and Darryl Veitch http://queue.acm.org/detail.cfm?id=1773943

Theo Schlossnagle is the founder and chief executive officer at Circonus, where he works on large-scale numerical data analysis. He is the author of Scalable Internet Architectures (Sams Publishing, 2006) and founder of Omni TI, an Internet consultancy.