river-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Gregg Wonderly <gr...@wonderly.org>
Subject Re: Possible multi-threading bug
Date Thu, 02 Dec 2010 14:52:24 GMT
In this kind of software design, where we have a reset(), get() and mutate() 
kind of method, the importance of volatile performance vs synchronized control 
is pretty much a part of what mutate() does.  If mutate() is really set(), then, 
you just have and assignment (typically) and thus there is no atomic view 
problem between get() and set().  get() will return the new value of set() at 
some point in the future, so volatile is enough in the class implementation.

However, in our case where mutate() involves, indirectly, a get() operation, 
then synchronized is required on all mutations because the total order of the 
involved operations is important, as Peter said in his post.  It can be that the 
references also need to be synchronized if there is an intermediate value that 
might be set() before the final mutate() action occurs.

Because of all of these considerations, it is usually far better to pick 
synchronized first until volatile can be proven to be more desirable.  That way, 
any future changes to the reference and mutator methods will not expose a new 
bug related to operation ordering or intermediate values.

Sometimes, people write methods like

	public void increment() {
		if( ++count > MAX ) {
			count = 0;
		}
	}

and there is thus two "set" operations and the final result is really all that 
you want to expose.  So synchronized here is a good idea.

A second issue is that if you are designing a "container" class that others 
might use in multiplicity such that it could end up in a container, and perhaps 
as a key data structure, it can be better to create an internal locking object 
to perform synchronization on so that if the user of your class starts using 
your class as a locking object, the internal locking does not impact unrelated 
code paths.

Gregg Wonderly


On 11/29/2010 5:45 AM, Tom Hobbs wrote:
> Yes, you're right.
>
> I knew about the non-atomicity of ++, my concern was a call to reset
> creeping in between the two parts of that operation.
>
> I forgot about the "Get a global lock on the variable" that volatile
> would require as mentioned here;
> http://www.javaperformancetuning.com/news/qotm051.shtml
>
> On Mon, Nov 29, 2010 at 10:50 AM, Peter Firmstone<jini@zeus.net.au>  wrote:
>> Tom Hobbs wrote:
>>>
>>> Serious comment:
>>>
>>> Shouldn't reset also be synchronized?
>>
>> You've identified that reset is also a mutator method, I guess I gave
>> mutation as my reason and I wasn't being entirely accurate, sorry.
>>
>> To explain a bit more, reset doesn't have to be synchronized when _count is
>> volatile and increment synchronized, the reason, it's a single operation
>> mutator, it just resets the counter, it doesn't check it first.
>>
>> The increment method is at least two operations _count++ first gets _count,
>> then increments it and sets _count to the new value, if it wasn't
>> synchronized, another increment could get _count at the same time... Or a
>> reset operation could occur in between and the reset fail.
>>
>> The ++ operator isn't atomic.
>>
>> If in doubt use synchronization, so by making reset also synchronized you
>> wouldn't be wrong and it probably wouldn't hurt performance either.
>>
>> The secret to concurrency is, all operations must happen atomically.
>>   Synchronization creates atomicity when it wouldn't otherwise exist.
>>   Synchronization also effects visibility, as does volatile.
>>
>> Cheers,
>>
>> Peter.
>>
>>>   Also, +1 for making _count volatile.
>>>
>>> Frivolous comment:
>>>
>>> Can we drop the underscore prefix for member variables?  It's against
>>> the official Sun/Oracle/Whatever conventions.  Also, it drives me up
>>> the wall...
>>>
>>> http://www.oracle.com/technetwork/java/codeconventions-135099.html#367
>>>
>>>
>>> On Sun, Nov 28, 2010 at 11:16 PM, Patricia Shanahan<pats@acm.org>  wrote:
>>>
>>>>
>>>> In the way in which this is used, I expect most of the calls to be to
>>>> increment. It has to be synchronized, so it seems simplest to synchronize
>>>> all the methods.
>>>>
>>>> I've done some more experiments, and decided this is a real problem. As
>>>> part
>>>> of my debug effort, I increased the number of threads in the stress test,
>>>> so
>>>> that it fails much more often. I also added some debug printouts, one of
>>>> which was showing up in conjunction with some but not all failures, so I
>>>> thought it was irrelevant.
>>>>
>>>> With the additional synchronization, the debug message shows up in all
>>>> failures. I think I actually had two forms of failure, one of which is
>>>> fixed
>>>> by the synchronization.  In the failure case that has been fixed,
>>>> everything
>>>> works, no debug messages, but the test never admits to having finished,
>>>> exactly the symptom I would expect from this issue.
>>>>
>>>> I plan to check in the enhanced test as well as the fixes, because it
>>>> only
>>>> takes a few minutes longer than the current size, and is much better at
>>>> finding bugs.
>>>>
>>>> Patricia
>>>>
>>>>
>>>> On 11/28/2010 2:52 PM, Peter Firmstone wrote:
>>>>
>>>>>
>>>>> Well, at the absolute minimum the variable should be volatile, so any
>>>>> changes are visible among all threads.
>>>>>
>>>>> Since increment is the only mutating method, this must be synchronized.
>>>>>
>>>>> This is a cheap form of multi read, single write threading, although
one
>>>>> must be careful, as this only works on primitives and immutable object
>>>>> references, since only the reference itself is being updated.
>>>>>
>>>>> If it was a reference to a mutable object (or long), then all methods
>>>>> would need to be synchronized.
>>>>>
>>>>> Cheers,
>>>>>
>>>>> Peter.
>>>>>
>>>>> Patricia Shanahan wrote:
>>>>>
>>>>>>
>>>>>> I've found something I think is a problem in
>>>>>> com.sun.jini.test.impl.outrigger.matching.StressTest, but it does
not
>>>>>> seem to be the problem, or at least not the only problem, causing
the
>>>>>> test hang I'm investigating. It's difficult to test, so I'd like
a
>>>>>> review of my reasoning. This is a question for those who are familiar
>>>>>> with the Java memory model.
>>>>>>
>>>>>> Incidentally, if we went to 1.5 as the oldest supported release,
this
>>>>>> could be replaced by an AtomicInteger.
>>>>>>
>>>>>> In the following inner class, I think the methods reset and getCount
>>>>>> should be synchronized. As the comments note, the operations they
>>>>>> perform are atomic. My concern is the lack of a happens-before
>>>>>> relationship between those two methods and the increment method.
As
>>>>>> far as I can tell, there is nothing forcing the change in the counter
>>>>>> due to an increment to become visible to a getCount call in another
>>>>>> thread.
>>>>>>
>>>>>> private class Counter {
>>>>>>
>>>>>> /**
>>>>>> * Internal counter variable.
>>>>>> */
>>>>>> private int _count = 0;
>>>>>>
>>>>>> /**
>>>>>> * Constructor. Declared to enforce protection level.
>>>>>> */
>>>>>> Counter() {
>>>>>>
>>>>>> // Do nothing.
>>>>>> }
>>>>>>
>>>>>> /**
>>>>>> * Resets internal counter to zero.
>>>>>> */
>>>>>> void reset() {
>>>>>>
>>>>>> // Integer assignment is atomic.
>>>>>> _count = 0;
>>>>>> }
>>>>>>
>>>>>> /**
>>>>>> * Increments internal counter by one.
>>>>>> */
>>>>>> synchronized void increment() {
>>>>>> ++_count;
>>>>>> }
>>>>>>
>>>>>> /**
>>>>>> * Returns current value of this<code>Counter</code> 
object.
>>>>>> */
>>>>>> int getCount() {
>>>>>>
>>>>>> // Returning an integer is atomic.
>>>>>> return _count;
>>>>>> }
>>>>>> }
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>


Mime
View raw message