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

Nathan Reynolds nathan.reynolds at oracle.com
Tue Apr 30 12:17:45 EDT 2013

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 
> <mailto: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
>>     <mailto: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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