RFR: 8040140 System.nanoTime() is slow and non-monotonic on OS X

David Holmes david.holmes at oracle.com
Tue Apr 15 07:14:42 UTC 2014

Hi Staffan,

Generally looks okay.

os_bsd.cpp still shows the old URL for Dave Dice's article


In the Solaris changes there is a lot of old code with inaccurate 
comments, but I suppose cleaning that up (oldgetTimeNanos()) is out of 
scope. You only added the check for AssumeMonotonicOSTimers in the 
supports_cx8 path, but the other path is now dead code.


Do we need to document this only affects OSX and Solaris? (Though 
implicitly this acts as-if true on Linux and Windows in the common case.)


   #ifdef TARGET_OS_FAMILY_bsd
   # include "jvm_bsd.h"
   # include <setjmp.h>
+ # include <mach/mach_time.h>

I think this include needs to be in a OSX/Apple specific conditional.


We should really fix the non-monotonic-clock path in the Linux and 
Windows implementations too ... but 32-bit is problematic <sigh>


On 15/04/2014 4:00 PM, Staffan Larsen wrote:
> Here is an updated webrev with changes to the comments in os_bsd.cpp and
> os_solaris.cpp.
>   - obs -> obsv
>   - fixed URL to blog entry
> http://cr.openjdk.java.net/~sla/8040140/webrev.01/
> /Staffan
> On 15 apr 2014, at 07:52, Staffan Larsen <staffan.larsen at oracle.com
> <mailto:staffan.larsen at oracle.com>> wrote:
>> On 14 apr 2014, at 21:08, Aleksey Shipilev
>> <aleksey.shipilev at oracle.com <mailto:aleksey.shipilev at oracle.com>> wrote:
>>> On 04/14/2014 06:55 PM, Staffan Larsen wrote:
>>>> mach_absolute_time() is essentially a direct call to RDTSC, but with
>>>> conversion factor to offset for any system sleeps and frequency
>>>> changes. The call returns something that can be converted to
>>>> nanoseconds using information from mach_timebase_info(). Calls to
>>>> mach_absolute_time() do not enter the kernel and are very fast. The
>>>> resulting time has nanosecond precision and as good accuracy as one
>>>> can get.
>>> Some numbers would be good on the public list :) I know the numbers
>>> already, but others on this list don’t.
>> I posted the numbers in the bug, but forgot to say so here...
>>>> Since the value from RDTSC can be subject to drifting between CPUs,
>>>> we implement safeguards for this to make sure we never return a lower
>>>> value than the previous values. This adds some overhead to nanoTime()
>>>> but guards us against possible bugs in the OS. For users who are
>>>> willing to trust the OS and need the fastest possible calls to
>>>> System.nanoTime(), we add a flag to disable this safeguard:
>>>> -XX:+AssumeMonotonicOSTimers.
>>> I now wonder if this safeguard can produce a stream of exactly the same
>>> timestamps if local clock is lagging behind. But considering the
>>> alternative of answering the retrograde time, and the observation the
>>> current Mac OS X mach_absolute_time() *appears* monotonic, having this
>>> safeguard seems OK.
>>>> webrev: http://cr.openjdk.java.net/~sla/8040140/webrev.00/
>>>> bug: https://bugs.openjdk.java.net/browse/JDK-8040140
>>> This looks good to me.
>> Thanks.
>>> And, since this question will inevitably pop up, do we plan to bring it
>>> into 8uX? I think many Mac users will be happy about that.
>> I would like to do so, but I would also like to have it sit and bake
>> for a while in 9 before that. I think the 8u20 train has left the
>> station, but perhaps 8u40?
>> /Staffan

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