[concurrency-interest] when is safe publication safe?

Joe Bowbeer joe.bowbeer at gmail.com
Mon Apr 26 01:17:44 EDT 2010


While I'm accumulating questions...

Why is ThreadLocal not the preferred cache in this case?

Joe

On Sun, Apr 25, 2010 at 4:26 PM, Joe Bowbeer wrote:

> Jochen,
>
> What you are describing seems like a caching problem as much as it is about
> safe publication and/or dynamic languages.  The language runtime creates the
> (immutable) instances and publishes them to the cache, right?  The
> performance of the cache is the hot spot?
>
> So are you using something like MapMaker to implement the cache?
>
> http://code.google.com/p/google-collections/
>
> What are you using to hold off a t2 when t1 is in the process of publishing
> to the cache?  Some scheme involving a Future?
>
> Joe
>
>
> On Sun, Apr 25, 2010 at 8:48 AM, Jochen Theodorou wrote:
>
>> Doug Lea wrote:
>>
>>> On 04/25/10 05:31, Jochen Theodorou wrote:
>>>
>>>> As a first step, consider exactly what effects/semantics you want
>>>>> here, and the ways you intend people to be able to write conditionally
>>>>> correct Groovy code.
>>>>>
>>>>
>>>> People wouldn't have to write conditionally correct Groovy code. they
>>>> would write normal code as they would in Java (Groovy and Java are very
>>>> near).
>>>>
>>>
>>> It seems implausible that you could do enough
>>> analysis at load/run time to determine whether you need
>>> full locking in the presence of multithreaded racy initialization
>>> vs much cheaper release fences. This would require at least some
>>> high-quality escape analysis. And the code generated
>>> would differ both for the writing and reading callers.
>>>
>>
>> maybe I did explain it not good. Let us assume I have the Groovy code:
>>
>> 1+1
>>
>> Then this is really something along the lines of:
>>
>> SBA.getMetaClassOf(1).invoke("plus",1)
>>
>> and SBA.getMetaClassOf(1) would return the meta class of Integer. Since
>> this is purely a runtime construct, it does not exist until the first time
>> this meta class is requested. So getMetaClassOf would be the place to
>> initialize the meta class, that would register it in a global structure and
>> on subsequent invocation use that cached meta class. If two threads execute
>> the code above, then one would do the initialization, while the other has to
>> wait. The waiting thread would then read the initialized global meta class.
>> On subsequent invocations both threads would just read. Since changes of the
>> meta class are rare, we would in 99% of all cases simply read the existing
>> value. Since we have to be memory aware, these meta class can be unloaded at
>> runtime too. They are SoftReferenced so it is done only if really needed.
>> But rather than the normal change a reinitialization might be needed much
>> more often.
>>
>> As you see the user code "1+1" does contain zero synchronization code. The
>> memory barriers are all in the runtime. It is not that this cannot be solved
>> by using what Java already has, it is that this is too expensive.
>>
>>
>>  As I mentioned, an alternative is to lay down some rules.
>>> If people stick to the rules they get consistent (in the sense
>>> of data-race-free) executions, else they might not. And of
>>> such rules, I think the ones that can apply here amount
>>> to saying that other threads performing initializations cannot
>>> trust any of their reads of the partially initialized object.
>>> And further, they cannot leak refs to that object outside of the
>>> group of initializer threads.
>>>
>>> This is not hugely different than the Swing threading rules
>>> (http://java.sun.com/products/jfc/tsc/articles/threads/threads1.html)
>>> but applies only during initialization.
>>>
>>
>> but unlike what the above may suggest there is no single initialization
>> phase. The meta classes are created on demand. We cannot know beforehand
>> which meta classes are needed and doing them all before starting would
>> increase the startup time big times.
>>
>> If there were of course a way to recognize a partially initialized object
>> I could maybe think of something... but is there a reliable one?
>>
>>
>> bye blackdrag
>>
>> --
>> Jochen "blackdrag" Theodorou
>> The Groovy Project Tech Lead (http://groovy.codehaus.org)
>> http://blackdragsview.blogspot.com/
>>
>>
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