[concurrency-interest] tsc register

David Holmes davidcholmes at aapt.net.au
Wed Jan 11 23:58:30 EST 2012


Hotspot uses the available OS high-resolution nominally monotonic time source if it exists, else it falls back to a time-of-day source (which is not monotonic). It should be very rare (ie only really old systems) to not have a monotonic timesource available.

Solaris had a number of bugs in this area (because unlike the other OSes that dropped use of the TSC due to its instability, Solaris decided to force it to be stable and synchronized - and occasionally they failed) and so a guard was added to ensure it was actually monotonic.

On Windows if the TSC is being used without using the external utilities/drivers to sync it then QueryPerformanceCounter can be non-monotonic. Similarly on Linux if you set your clocksource to be TSC instead of HPET (and the TSC is not synchronized) then CLOCK_MONOTONIC can also exhibit non-monotonic behaviour.

See bug 6458294 for some info. Sadly, back in November 2006 I reported that we would add the guard logic on all platforms, but it never happened.

All-in-all clocks/counters/timers are a general mess.

David

-----Original Message-----
From: concurrency-interest-bounces at cs.oswego.edu [mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of Dr Heinz M. Kabutz
Sent: Wednesday, 11 January 2012 8:33 AM
To: dholmes at ieee.org
Cc: Concurrency-interest at cs.oswego.edu
Subject: Re: [concurrency-interest] tsc register


  What is interesting is that we have had reports of System.nanoTime() sometimes counting backwards.  Has nanoTime() always been monotonic?  If so, I need to follow up on the claims.  I've heard it from two sources, but it might just be hearsay.

Regards

Heinz
-- 
Dr Heinz M. Kabutz (PhD CompSci)
Author of "The Java(tm) Specialists' Newsletter"
Sun Java Champion
IEEE Certified Software Development Professional
http://www.javaspecialists.eu
Tel: +30 69 72 850 460
Skype: kabutz 


  On 1/11/12 12:14 AM, David Holmes wrote: 
    Correct. Hotspot uses/relies-on the high-resolution monotonic time source of the OS, else falls back to plain time-of-day. It never uses the TSC directly.

    David Holmes
      -----Original Message-----
      From: concurrency-interest-bounces at cs.oswego.edu [mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of Vitaly Davidovich
      Sent: Wednesday, 11 January 2012 2:28 AM
      To: Nathan Reynolds
      Cc: Concurrency-interest at cs.oswego.edu
      Subject: Re: [concurrency-interest] tsc register


      I thought JVM (hotspot at least) uses the os monotonic clock source (if present) rather than reading tsc directly and then doing its own adjustments? 

      On Jan 10, 2012 11:24 AM, "Nathan Reynolds" <nathan.reynolds at oracle.com> wrote:

        The tsc register on older processors did not increment at the same rate.  If a core slept or slowed down then the tsc register would stop or slow down its increments.  More modern processors guarantee that tsc register increments at a fixed frequency.  If you are working on Linux, cpuinfo (?) could report the const_tsc flag.  This means that the processor and OS recognize that this feature is on the processor.

        The tsc register is not synchronized across sockets.  This is something Oracle has asked Intel to enhance many times.  It is a very difficult problem to solve.  However, more modern Linux kernels will (?) synchronize the tsc register at startup so that it is impossible to read the tsc register on two different cores and see that the 2ⁿᵈ value is smaller.  This does not mean that the tsc register is synchronized.  It only means that two threads running on different cores will hopefully never see the tsc "move backwards".

        There is no guarantee that once the tsc register is synchronized across sockets that it will remain so.  Some processors are hot swappable.  The newly added processor is not going to have the correct tsc register value.  Furthermore, the OS is free to reset the tsc value at any time.

        If I understand correctly, the HotSpot JVM will guarantee that System.nanoTime() never moves backwards.  It reads the tsc register with each call (?).  It the compares the read value with the last read value.  If the read value is < the last read value, then the last read value is returned.  If the read value is > the last read value, then the last read value is updated and the read value is returned.  Updating the last read value requires a CAS.  This CAS can lead to scalability bottlenecks if System.nanoTime() is called too frequently.  I am not sure if a better algorithm has been devised to fix this CAS contention.  I kind of remember it being talked about.

        I think the JVMs will default to more stable clock sources with worse resolution for nanoTime() if tsc is not behaving well.


        Nathan Reynolds | Consulting Member of Technical Staff | 602.333.9091
        Oracle PSR Engineering | Server Technology


        On 1/10/2012 5:03 AM, Dr Heinz M. Kabutz wrote: 
Only if you use System.nanoTime().  Time difference might even be
negative if the thread is swapped between different cores.

On 10/01/2012, Mohan Radhakrishnan <radhakrishnan.mohan at gmail.com> wrote:
        Hi,

One more question from the novice and for the novice.

I see these points in Dr. click's PPT. Can I know why ? I ask this
here because it seems to
involve multiple cores. Maybe the jvm forums are better suited for this.
Does this mean that we get wrong time values if threads run on
different cores ?

But cannot use, e.g. X86's "tsc" register
? Value not coherent across CPUs
? Not consistent, e.g. slow ticking in low-power mode
? Monotonic per CPU – but not per-thread

Thanks,
Mohan

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