[infinispan-dev] Eviction overhaul

[Mircea Markus]

I see.
Now, we have a data container in which we are assured (by LockManager, 
right?) that we won't have concurrent writes on the same key.
I'm just thinking weather we need an CHM or something less (simple 
HashMap?) for assuring consistency.
Cheers,
Mircea

Manik Surtani wrote:
>
> On 2 Apr 2009, at 09:10, Mircea Markus wrote:
>
>> Manik Surtani wrote:
>>> Hello all.
>>>
>>> I have finished my work with the eviction code in Infinispan, here is a 
>>> summary of what has happened.
>>>
>>> From a user perspective (including API and configuration) as well as 
>>> a design overview, please have a look at
>>> http://www.jboss.org/community/docs/DOC-13449
>>>
>>> From an implementation perspective, have a look at the srcs of 
>>> FIFODataContainer and LRUDataContainer.  These two classes are where 
>>> everything happens.  The javadocs should explain the details, but in 
>>> a nutshell you can expect constant time operations for all puts, 
>>> gets, removes, iterations.  :-)  
>>> Feedback on the impls would be handy.  :-)
>> I like to concept - it's nice and clear for users to grasp. Also took 
>> a look at implementation, nice and complex, so I'd rather have you 
>> and a white board to understand it completely :) .
>> Some notes though: EvictionException is never used, same for 
>> SpinLock.rl field.
>
> Good point, EvictionException was a carry-over from JBC which can go away.
>
> SpinLock.rl was something I used for debugging/tracking race 
> conditions - it should go away now as well.
>
>> And a question:
>> with this impl, we're doing locking twice for cache operations: on 
>> LockInterceptor and on DataContainer itself. Why is this necessary? I 
>> think this is needed so that eviction thread and user threads not to 
>> conflict( am I right?).
>
> Well, the locking is done on a different level.  The 
> LockingInterceptor locks a key.  So keep in mind that 2 threads may 
> lock 2 separate keys concurrently, but these may map to the same hash 
> bucket in the data container.  In which case, since the DC uses a CHM, 
> this would mean that the same stripe is locked preventing concurrent 
> reorganisation of the stripe.
>
>> Isn't it possible to use the same locking (i.e. LockManager) for both 
>> these threads?
>
> Possible, but again not ideal, for example if you have TX 1 which 
> writes to key K1.  The way this currently works, here is what happens:
>
> 1.  Acquire lock for key K1 in the lock manager
> 2.  Write to K1
> 3.  Sleep?
> 4.  Commit:
> 5.  Write changes to DC.  This will lock the stripe in the CHM for the 
> duration of this step only.
> 6.  Release lock acquired in 1
>
> Now if you have a concurrent write to K2, which *happens* to be in the 
> same stripe as K1 in the CHM, this second thread will *not* block for 
> TX 1 since TX 1 only locks the CHM stripe for a short duration (step 5 
> above).  
>
> Now if we used the same locks thoughout (e.g., step 1 *is* the lock 
> stripe in the CHM) then you have far less concurrency.
>
> Of course, the above premise only holds true if you have enough shared 
> locks for the lock manager such that K1 and K2 do not map to the same 
> lock, even if they are in the same stripe in the CHM.  Also holds true 
> if lock striping is disabled and you have a lock per key.
>
>> (also, why are we using  ConcurrentHashMaps in default container - 
>> isn't access to data guarded by the lock interceptor?
>
> See above.  :-)
>
> Cheers
> Manik
>
> --
> Manik Surtani
> Lead, JBoss Cache
> http://www.jbosscache.org
> manik at jboss.org <mailto:manik at jboss.org>
>
>
>
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