[concurrency-interest] ThreadLocal vs ProcessorLocal

Jacy Odin Grannis jacyg at alumni.rice.edu
Wed Oct 17 23:46:53 EDT 2012


So, when I'd seen this it was on Linux 2.6.9, Opteron 8384s, Java
1.6.0_16 (it was a couple years back).

Based on this:
http://juliusdavies.ca/posix_clocks/clock_realtime_linux_faq.html  it
would look like it could be that the kernel was buggy, as only 2.6.18
and up are solid.

However, I also found this link:
http://efreedom.com/Question/1-6814792/Clock-Gettime-Erratic  which
notes erratic behavior even on 2.6.26, when using an Opteron.
Somewhere in this thread
http://stackoverflow.com/questions/510462/is-system-nanotime-completely-useless
someone comments that when they used AMD before that there wasn't any
synchronization even across cores on the same die.  So, I'm going to
guess AMD was the source of the behavior I saw.  The values were
definitely very different, they would frequently be separated by
hundreds of thousands of "nanos" from one core to the next.  Makes for
funny outliers in your data set when you're trying to capture timings.
 Although my big concern wasn't the huge swings--those you can easily
discard--it was worry that I'd end up with noise I couldn't account
for if the timings were close but not quite in sync.

At any rate, hopefully that's helpful to someone if they're trying to
debug similarly strange results on an older machine.

Jacy

On Wed, Oct 17, 2012 at 9:31 PM, David Holmes <davidcholmes at aapt.net.au> wrote:
> Well the nanoTime() behaviour is not a JVM bug, though the JVM can try to
> account for the underlying buggy OS and/or configuration and/or hardware.
> ;-)
>
> As Dave states on LInux we will use CLOCK_MONOTONIC if available (which is
> pretty much always these days), else we fall back to gettimeofday. Now as
> you can infer from the name CLOCK_MONOTONIC is supposed to be monotonic and
> if it isn't that is a bug in the OS or a system configuration error (using
> an unreliable clocksource such as the TSC on MP systems). In contrast we
> make no pretense that gettimeofday is expected to be monotonic.
>
> Also as Dave states we don't try to guard against a buggy CLOCK_MONOTONIC on
> linux by ensuring it never reports a value less than any previous value
> reported. We could, and probably should, but it is one of many things on a
> long list.
>
> But if you see big problems with nanoTime then either your system is using
> the TSC as a clocksource when it should not, OR you are running in a virtual
> environment and the host system is not providing a stable time source to the
> guest OS.
>
> Note: for the TSC to be usable it must be both stable (frequency invariant)
> and synchronized across all "processors". While many processors now provide
> a stable TSC they don't provide a synchronized TSC. For the OS to be able to
> use the TSC as a monotonic clocksource it needs to do its own very accurate
> synchronization. Solaris actually attempts this, where most operating
> systems simply stopped using the TSC, but because of that there has been a
> very long bug-tail on Solaris.
>
> David Holmes
>
> -----Original Message-----
> From: concurrency-interest-bounces at cs.oswego.edu
> [mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of David Dice
> Sent: Thursday, 18 October 2012 12:11 PM
> To: concurrency-interest at cs.oswego.edu
> Subject: Re: [concurrency-interest] ThreadLocal vs ProcessorLocal
>
>
>
>> Date: Wed, 17 Oct 2012 16:55:50 -0500
>> From: Jacy Odin Grannis <jacyg at alumni.rice.edu>
>> To: "Dr Heinz M. Kabutz" <heinz at javaspecialists.eu>
>> Cc: concurrency-interest at cs.oswego.edu, David Dice
>>         <david.dice at gmail.com>
>> Subject: Re: [concurrency-interest] ThreadLocal vs ProcessorLocal
>> Message-ID:
>>
>> <CAESiqEqAteCAsCDLWXM-3-89bJS2nBRrLtOBmHrJkkcX2=Sh4g at mail.gmail.com>
>> Content-Type: text/plain; charset=ISO-8859-1
>>
>> Yes, definitely.  I've seen this happen.  One easy way you can see
>> this is System.nanoTime will suddenly start returning wildly different
>> values.  nanoTime is only consistent on a single processor, it can
>> vary widely between processors (at least on Linux).
>>
>> I think what's really needed is a set of language level constructs for
>> really addressing the problem.  I know there are experimental projects
>> looking to do that (
>>
>> https://wiki.rice.edu/confluence/display/HABANERO/Habanero+Multicore+Software+Research+Project
>> ).  I am not sure to what extent it would be possible to build support
>> for the various constructs in the JVM; and then aside from that, how
>> you would add language support is another matter.
>
>
> The nanoTime() behavior sounds like a JVM bug.   nanoTime() values should be
> non-retrograde and causal in the sense that if one thread calls nanoTime and
> stores the observed value T into a variable, and then some 2nd thread reads
> that variable and observes the store of T and then calls nanoTime and sees
> value U, we should have U >= T.  (Volatiles are assumed, obviously).     I
> first ran into this non-monotonic time problem on large SPARC systems where
> the HW clock underlying the native gethrtime() API exhibited drift between
> CPUs.   The drift was minimal as the kernel syncs the clocks periodically,
> so we tracked the the maximum value returned by nanoTime() and would return
> the maximum of that tracking value and the value we got via gethrtime().
> This works, but creates its own cache coherence hot-spot as we're updating
> that variable frequently, which means that concurrent and unrelated
> nanoTime() calls don't scale as well as we might like.   (There are ways to
> avoid the coherence hot spot but they usually entail reduced accuracy).
>
> It's been years since I've looked at the code, but I think we use
> CLOCK_MONOTONIC if it's available on linux.   (David Holmes could best
> answer this part of the question regarding linux time sources).  But the
> guards against returning a smaller value aren't in place in the linux
> platform-specific code as they are on Solaris.
>
> Regards
> Dave
>
>
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