[hibernate-dev] HSEARCH-189 & concurrency

[Sanne Grinovero]

Thanks for your blog Pavitar;
I would like to add some clarification about the "piggyback" just to
confirm:
there is no such concept as "THE shared memory" in the JMM, shared memory is
about memory shared between some threads, not necessarily all.
the "piggyback" trick works basing on this: after ThreadB reads a volatile
variable, it is guaranteed to see at least ALL state TrheadA had written
before (and during)
a write to THAT SAME volatile variable. Note the "at least" wording: more
changes could happen to the other variables after the write to the volatile,
and some (in no order,
especially not the code order) could be also seen, but no guarantee about.
so you could:
write field1, field2, field3 and then the volatile field4 in T1
read volatile field4, field1, field2, field3 in T2 (after T1 did)
and you will get a guarantee that T2 will "see" at least the state written
by T1.

So this is a "trick" to avoid longer locks or having to convert them all to
volatile, but IMHO
the code is made difficult to maintain, and tricky to get it right.

In our practical case:
you could write to some volatile field in the SearchFactoryImpl after the
initialization is done,
but then you still have to ensure all subsequent uses will read the same
field before anything else;
this has a minimal impact on performance, the good think about the
"piggyback" is you
could use a read to a volatile you would have anyway.
I don't think this is at all needed for the SearchFactoryImpl as long as you
guarantee that
the threads going to use it are started ("start()") after and by the
initialization thread;
if this is correct no further discussion is needed there.
In the readeProviders and DPProviders I can't get this guarantee, that's why
they need a fix.

Shall I use this trick then? It isn't so bad if you think it's accepatable
to use it, it's good for performance
but I dislike it for code readability; I'll add a big fat scary warning.
IMHO this should be avoided when possible, especially since "final" works
fine and is
very explicit to another code reader.

If you think I should go for the pig, I would appreciate if Pavitar could
read the code after I commit it
to check my code, even if this case is trivial.

Sanne

2008/7/4 Emmanuel Bernard <emmanuel at hibernate.org>:

> Hey,Can you tell me more about the piggyback synchronization. I could not
> find any decent knowledge online.
> how far reading a volatile guarantee that all "local" values of the thread
> we are reading from will be pushed to the shared memory?
> For example, could reading a volatile value after HSearch is done with
> initialization (all init is done in a single thread) guarantee that all
> states held by this thead will be pushed back to the shared memory?
> The use case is quite specific, I init everything in a single thread, want
> to push all the state to the shared memory. I know post init() use of
> HSearch will never change the state so I don't "need" locking.
>
> --
> Emmanuel Bernard
> http://in.relation.to/Bloggers/Emmanuel | http://blog.emmanuelbernard.com|
> http://twitter.com/emmanuelbernard
> Hibernate Search in Action (http://is.gd/Dl1)
>
> On  Jul 4, 2008, at 04:33, Sanne Grinovero wrote:
>
> Hi Pavitar Singh,
>
> I thank you very much about your explanations but actually I opened the
> issue myself
> because I have read the same specs and am aware of that.
> in H.Search (and several other hibernate code) there's this quite common
> pattern for starting
> "replaceable" objects (something like user-written plugins, you can provide
> your own implementation
> to do some stuff) but this same pattern is also used to start the built-in
> default strategies.
>
> It looks like this:
> - an empty constructor, to use class.newInstance();
> - an initialize() to set configuration options
> - a start() method (eventually) used to start background tasks
> - some doStuff() and/or getXX() which need to be fast & threadsafe
>
> As you can see in Concurrecy in Practice at page 50, this is BAD, as for
> example in the FSSlaveDirectoryProvider nobody takes care of locking
> or visibility, and nobody is doing anywhere where I see this pattern used
> (several times in the project).
> I'm not saying it is all broken, because usually the threads consuming
> these unsafely-initialized objects are started after the initialization, so
> that's ok. In this specific case the state will be used to communicate
> between threads, so some visibility fix is needed.
>
> I know I could use it only for final fields, but this is exactly what I
> want:
> there are currently 10 instance variables, of these
> 4 have no concurrent use
> 4 are configuration constants and could use the "final" (they're not safely
> published)
> 2 would need some lock/volatile anyway, bot only one is used frequently, so
> IMHO 1 volatile is ok.
>
> I was thinking in using the same Piggyback technique you mention to
> safely publish the initialization constants,
> but I'm afraid the code will become more difficult to maintain and more
> "unreadable", possibly breaking at the first next patch:
> IMHO using some unchanging fields "final" is the most clean and
> readable solution (and best performing), but I need a different
> constructor for that.
>
> your opinion is really welcome as I couldn't find other feedback,
> if you would like to take a look at the sources download the Search trunk
> and look at:
> org.hibernate.search.store.FSSlaveDirectoryProvider
> or the FIXME in
> org.hibernate.search.reader.SharingBufferReaderProvider
>
> Sanne
>
> 2008/7/4 Pavitar Singh <pavitar.singh at gmail.com>:
>
>> Hi Sanne,
>>
>> I don't think moving everything in constructor can guarantee safe
>> publication.
>>
>> From the JMM Specification Section 3.5
>>
>> "An object is considered to be completely initialized when its constructor
>> finishes. A thread that can only see a reference to an object after that
>> object has been completely initialized is guaranteed to see the correctly
>> initialized values for that object's final fields."
>>
>> That means there are no guarantees for other fields which are non-final.
>>
>> But once things are moved in constructor then by using Safe Publication
>> one can enforce visibility guarantees.
>>
>> Other thing i was wondering was why do one need to make every field as
>> volatile. As just by using a single volatile variable one can enforce memory
>> barriers. A technique documented in JCIP as Piggyback Synchronizations and
>> used by Doug Lea in implementing ConcurrentHashMap.(You will find get method
>> in ConcurrentHashMap is without any locking and visibility is enforced by
>> use of a single volatile variable.)
>>
>> Also if you can elaborate more on how you are facing the visibility issue
>> then may be i can also spend time on it on figuring performant solution.
>>
>> Regards
>> Pavitar Singh
>> http://pitfalls.wordpress.com
>>
>> On Fri, Jul 4, 2008 at 5:13 AM, Sanne Grinovero <
>> sanne.grinovero at gmail.com> wrote:
>>
>>> Hello all,
>>> I'm sorry I've been very busy but as promised I would like to fix
>>> HSEARCH-189
>>> (and others) very soon;
>>>
>>> I would like to propose a API extension (backwards compatible) that would
>>> simplify the patch a lot:
>>> keeping it as is it is very tricky to fix the visibility issues in
>>> FSSlaveDirectoryProvider
>>> and FSMasterDirectoryProvider without introducing a performance penalty.
>>>
>>> I have these options to close the issue:
>>> 1) add a "volatile" to more than six fields per class (ugly and not good
>>> for performance)
>>> 2) use some Locks/synch (more readable, still performance hits)
>>> 3) move the "initialize" arguments to a constructor.
>>>
>>> As Emmanuel knows I would really love the third option, but he's worried
>>> about
>>> the fact we can't force a constructor in an interface*1, so my proposal
>>> is:
>>>
>>> if we find there exists a constructor having the same arguments as the
>>> initialize method,
>>> we use that, otherwise we use a no-arg constructor and then call the
>>> initialize.
>>>
>>> Reflection is used anyway to instantiate these components,
>>> the code in DirectoryProviderFactory doesn't get much more complicated
>>> and much more cleanup is made possible in all DPs because of this
>>> (as the equals/hashcode comments already ask for).
>>>
>>> I actually think this same pattern is needed for other components,
>>> such as all ReaderProvider, so I hope you'll want to give it a try
>>> and let me apply it on other components too if you like the resulting
>>> code.
>>>
>>> Sanne
>>>
>>> _______________________________________________
>>> hibernate-dev mailing list
>>> hibernate-dev at lists.jboss.org
>>> https://lists.jboss.org/mailman/listinfo/hibernate-dev
>>>
>>>
>>
>>
>> --
>> Pavitar Singh
>> Blog: http://pitfalls.wordpress.com
>
>
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