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 <rva...@redhat.com> JBoss Performance Team _______________________________________________ infinispan-dev mailing list infinispan-dev@lists.jboss.org https://lists.jboss.org/mailman/listinfo/infinispan-dev
