Semyon, could you please give a link to JIRA issue (if any) and what branch contains your code?
Also it is not clear for me, how transaction order is assigned / calculated? If I start transaction t1 on none n1 and t2 on node n2, how it will be calculated? Thanks. On Thu, Oct 15, 2015 at 2:00 PM, Semyon Boikov <[email protected]> wrote: > Hello, > > I'm working on new implementation for optimistic/serializable transaction > mode (current implementation is inefficient and have bugs). This mode is > supposed to be used when concurrent transactions do not update the same > keys, in this case transactions can be executed more efficiently, but if > concurrent transactions have read of write conflicts then > TransactionOptimisticException can be thrown and transaction should be > retried. Also with current transactions implementation user should order > updated keys, otherwise deadlock is possible, we want to remove this > requirement for optimistic/serializable mode. > > Issue with read/write conflicts can be detected if when read is performed > entry version is stored and then compared with current version when > transaction lock is acquired. > > Another issue is avoid deadlocks when transactions acquire keys in > different order. > > I created one possible solution using 'try-lock' approach: for each cache > entry we already have queue with current lock owner and transactions trying > to acquire entry lock. > When optimistic/serializable transaction tries to acquire entry lock it > checks that entry is not locked and there are no others transactions > waiting for lock entry, otherwise transaction fails with > TransactionOptimisticException. But this approach does not work well when > two transaction have lot of intersecting keys: it is possible that these > transaction will constantly conflict and will both constantly fail with > TransactionOptimisticException. > > It seems there is better approach to resolve these conflicts to do not fail > all conflicting transactions: > - we should order all transactions by some attribute (e.g. all transactions > already have unique version) > - transaction with greater order should always 'win' transaction with lower > order > - per-entry queue with waiting transactions should be sorted by this order > - when transaction tries to acquire entry lock it is added in waiting queue > if queue is empty or last waiting transaction have lower order, otherwise > transaction fails > > With this approach transaction lock assignment is ordered and transactions > with lower order never wait for transactions with greater order, so this > algorithm should not cause deadlocks. > > I also created unit test simulating this algorithm and it did not reveal > any issues. Also in this unit tests I measured percent of rollbacks when > concurrent updates have lots of conflicts: with 'try-lock' approach percent > of rollbacks is ~80%, with new algorithm is ~1% (but of course with > real-life test results can be different). > > Does anyone see problems with this locking approach? > -- Alexey Kuznetsov GridGain Systems www.gridgain.com
