[concurrency-interest] tsc register

David Holmes davidcholmes at aapt.net.au
Tue Jan 10 17:14:34 EST 2012

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:

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


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