[infinispan-dev] Fine grained maps

[Radim Vansa]

Hi all,

I have realized that fine grained maps don't work reliably with 
write-skew check. This happens because WSC tries to load the entry from 
DC/cache-store, compare versions and store it, assuming that this 
happens atomically as the entry is locked. However, as fine grained maps 
can lock two different keys and modify the same entry, there is a risk 
that the check & store won't be atomic. Right now, the update itself 
won't be lost, because fine grained maps use DeltaAwareCacheEntries 
which apply the updates DC's lock (there can be some problems when 
passivation is used, though, [1] hopefully deals with them).

I have figured this out when trying to update the DeltaAware handling to 
support more than just atomic maps - yes, there are special branches for 
atomic maps in the code, which is quite ugly design-wise, IMO. My 
intention is to do similar things like WSC for replaying the deltas, but 
this, obviously, needs some atomicity.

IIUC, fine-grained locking was introduced back in 5.1 because of 
deadlocks in the lock-acquisition algorithm; the purpose was not to 
improve concurrency. Luckily, the days of deadlocks are far back, now we 
can get the cluster stuck in more complex ways :) Therefore, with a 
correctness-first approach, in optimistic caches I would lock just the 
main key (not the composite keys). The prepare-commit should be quite 
fast anyway, and I don't see how this could affect users 
(counter-examples are welcome) but slightly reduced concurrency.

In pessimistic caches we have to be more cautious, since users 
manipulate the locks directly and reason about them more. Therefore, we 
need to lock the composite keys during transaction runtime, but in 
addition to that, during the commit itself we should lock the main key 
for the duration of the commit if necessary - pessimistic caches don't 
sport WSC, but I was looking for some atomicity options for deltas.

An alternative would be to piggyback on DC's locking scheme, however, 
this is quite unsuitable for the optimistic case with a RPC between WSC 
and DC store. In addition to that, it doesn't fit into our async picture 
and we would send complex compute functions into the DC, instead of 
decoupled lock/unlock. I could also devise another layer of locking, but 
that's just madness.

I am adding Sanne to recipients as OGM is probably the most important 
consumer of atomic hash maps.

WDYT?

Radim

[1] 
https://github.com/infinispan/infinispan/pull/4564/commits/2eeb7efbd4e1ea3e7f45ff2b443691b78ad4ae8e

-- 
Radim Vansa <rvansa at redhat.com>
JBoss Performance Team