[concurrency-interest] JLS 17.7 Non-atomic treatment of double and long : Android

Stanimir Simeonoff stanimir at riflexo.com
Tue Apr 30 12:57:26 EDT 2013

On Tue, Apr 30, 2013 at 7:37 PM, Vitaly Davidovich <vitalyd at gmail.com>wrote:

> Curious how x86 would move a long in 1 instruction? There's no memory to
> memory mov so has to go through register, and thus needs 2 registers (and
> hence split).  Am I missing something?
> It uses ab SSE instruction, they are wider.


> Sent from my phone
> On Apr 30, 2013 12:23 PM, "Nathan Reynolds" <nathan.reynolds at oracle.com>
> wrote:
>>  You might want to print the assembly using HotSpot (and OpenJDK?).  If
>> the assembly, uses 1 instruction to do the write, then no splitting can
>> ever happen (because alignment takes care of cache line splits).  If the
>> assembly, uses 2 instructions to do the write, then it is only a matter of
>> timing.
>> With a single processor system, you are waiting for the thread's quantum
>> to end right after the first instruction but before the second
>> instruction.  This will allow the other thread to see the split write.
>> With a dual processor system, the reader thread simply has to get a copy
>> of the cache line after the first write and before the second write.  This
>> is much easier to do.
>> HotSpot will do a lot of optimizations on single processor systems.  For
>> example, it gets rid of the "lock" prefix in front of atomic instructions
>> since the instruction's execution can't be split.  It also doesn't output
>> memory fences.  Both of these give good performance boosts.  I wonder if
>> with one processor, OpenJDK is using 2 instructions to do the write whereas
>> with multiple processors it plays it safe and uses 1 instruction.
>> Note: If you disable all of the processors but 1 and then start HotSpot,
>> HotSpot will start in single processor mode.  If you then enable those
>> processors while HotSpot is running, a lot of things break and the JVM will
>> crash.  Because single processor systems are rare, the default might be
>> changed to assume multiple processors unless the command line specifies 1
>> processor.
>> Nathan Reynolds<http://psr.us.oracle.com/wiki/index.php/User:Nathan_Reynolds>| Architect |
>> 602.333.9091
>> Oracle PSR Engineering <http://psr.us.oracle.com/> | Server Technology
>>  On 4/30/2013 8:48 AM, Tim Halloran wrote:
>>  Aleksey, correct -- more trials show what you predicted. Thanks for the
>> nudge.
>>  Mark,
>>  Very helpful, in fact, we are seeing quick failures except for the
>> dual-processor case -- on a dual processor hardware or VM (Virtual Box) we
>> have yet to get a failure.  The two programs attached are what I'm running.
>>  I stripped out my benchmark framework (so they are easy to run on OpenJDK
>> but not on Android).  The difference is that one uses two threads (one
>> writer one reader) the other three (two writers one reader) -- both seem to
>> produce similar results.
>>  With one processor, OpenJDK 1.6.0_27 I see the split write almost
>> immediatly. Dual we can't get a failure, yet, we get more failures as the
>> processor count goes up -- but after a few failures, we don't get any more
>> (they program tries to get 10 to happen)...we can't get to 10.
>>  It seems that while this can happen on OpenJDK it is rarer than on
>> Android where ten failures takes less than a second to happen.
>>  Best, Tim
>> On Tue, Apr 30, 2013 at 11:26 AM, Mark Thornton <mthornton at optrak.com>wrote:
>>>   On 30/04/13 15:36, Tim Halloran wrote:
>>> On Mon, Apr 29, 2013 at 4:59 PM, Aleksey Shipilev <
>>> aleksey.shipilev at oracle.com> wrote:
>>>> Yes, that's exactly what I had in mind:
>>>>  a. Declare "long a"
>>>>  b. Ramp up two threads.
>>>>  c. Make thread 1 write 0L and -1L over and over to field $a
>>>>  d. Make thread 2 observe the field a, and count the observed values
>>>>  e. ...
>>>>  f. PROFIT!
>>>> P.S. It is important to do some action on value read in thread 2, so
>>>> that it does not hoisted from the loop, since $a is not supposed to be
>>>> volatile.
>>>> -Aleksey.
>>>  This discussion is getting a bit far afield, I guess, but to get back
>>> onto the topic. I followed Aleksey's advice. And wrote an implementation
>>> that tests this.  I used two separate threads to write 0L and -1L into the
>>> long field "a" but that is the only real change I made. (I already had some
>>> scaffolding code to run things on Android or desktop Java).
>>>  *Android: splits writes to longs into two parts.*
>>>  On a Samsung Galaxy II with Android 4.0.4  a Nexus 4 phone with
>>> Android 4.2.2 I saw non-atomic treatment of long. The value -4294967296
>>> (xFFFFFFFF00000000) showed up as well as 4294967295 (x00000000FFFFFFFF).
>>>  So looks like Android does not follow the (albeit optional) advice in
>>> the Java language specification about this.
>>>  *JDK: DOES NOT split writes to longs into two parts (even 32-bit
>>> implementations)*
>>>  Of course we couldn't get this to happen on any 64-bit JVM, but we
>>> tried it out under Linux on 32-bit OpenJDK 1.7.0_21 it does NOT happen. The
>>> 32-bit JVM implementations follow the recommendation of the Java language
>>> specification.
>>>  An interesting curio. I wonder how many crashes in "working" Java code
>>> moved from desktop Java onto Android programmers are going to lose sleep
>>> tracking down this one.
>>>  Last time I tried this sort of test, a split write would be observed
>>> in under a second on a true dual processor. However, with only one
>>> processor available, it would typically take around 20 minutes. So you
>>> might have to run a very long test to have any real confidence in the lack
>>> of splitting.
>>> Mark Thornton
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