Mac OS X System.nanoTime Improvements

[David Holmes]

On 15/05/2013 6:44 AM, Денис Ушаков wrote:
> Hi,
>
> couple of days ago Aleksey Shipilev mentioned that System.nanoTime() is very slow on OS X and is non-monotonic. After some research I found that there's no clock_gettime() function in OS X and current implementation falls back to gettimeofday().

Wow! If that is the case that is truly horrible and the OSX porters 
should have been aware of that and flagged it. :( A nanoTime() based on 
gettimeofday would not be monotonic if ntp adjustments are being made.

> OS X provides a monotonic clock function mach_absolute_time(). According to documentation (https://developer.apple.com/library/mac/#qa/qa1398/_index.html) it can be converted to nanoseconds using results of mach_timebase_info().

We need to know more about how this works, and in particular whether it 
is using the TSC and so is subject to the well known problems using the 
TSC as a "clock" base. The API sounds similar to that on Windows: 
QueryPerformanceCounter + QueryPerformanceFrequency.

Thanks,
David
-----

> Here's draft patch that uses mach_absolute_time() to provide fast and monotonic System.nanoTime() for OS X:
> https://gist.github.com/denofevil/db57fa547510b2dd87fd
>
> You can find benchmark written by Aleksey Shipilev here: https://github.com/shipilev/jmh-nanotime.
> Benchmark results with system specs: https://gist.github.com/denofevil/5026294ee6de42699b41
>
> Thanks,
> Dennis
>