Re: [infinispan-dev] The "Triangle" pattern for reducing Put latency
On 11/27/2015 11:12 AM, Radim Vansa wrote:
The update needs to be applied to *all* owners before the call
returns
on O. With your strategy, P could apply update, send ACK but the async
backup updates would not be delivered on Bs; so an ACKed update would
get completely lost.
Err, I forgot to say that P crashes after sending the ACK, but before
making sure that updates are delivered to B.
One thing about relying on JGroups FIFO is that as it's not per-key,
single message lost on network will delay *all* other updates before the
resend kicks in and sorts it out. That's not a very appealing
characteristics, and as we seem to be in need for versions on entries
for different purposes as well, I would rather do the ordering in
Infinispan and let JGroups handle only the reliability.
R.
I don't say that these async Bs are not possible, but not in the
basic
case - for default configuration, we need to keep the guarantees.
Radim
On 11/27/2015 10:34 AM, Bela Ban wrote:
> Adding to what Radim wrote (below), would the following make sense
> (conditions: non-TX, P != O && O != B)?
>
> The lock we acquire on P is actually used to establish an ordering for
> updates to the Bs. So this is very similar to SEQUENCER, expect that we
> have a sequencer (P) *per key*.
>
> Phase 1
> -------
> - O sends a PUT(x) message to P
>
> Phase 2
> -------
> - P adds PUT(x) to a queue and returns (freeing the up-thread)
> - A thread dequeues PUT(x) and sends an (async) UPDATE message to all Bs
> (possible optimization: send updates to the same key sets as
> batches)
> - PUT(x) is applied locally and an ACK is sent back to O
>
> O times out and throws an exception if it doesn't receive the ack
> from P.
>
> This would reduce the current 4 phases (for the above conditions) to 2,
> plus the added latency of processing PUT(x) in the queue. However, we'd
> get rid of the put-while-holding-the-lock issue.
>
> P's updates to the Bs are FIFO ordered, therefore all we need to do is
> send the update down into UNICAST3 (or NAKACK2, if we use multicasts)
> which guarantees ordering. Subsequent updates are ordered according to
> send order. The updates are guaranteed to be retransmitted as long as P
> is alive.
>
> If P crashes before returning the ack to O, or while updating the Bs,
> then O will time out and throw an exception. And, yes, there can be
> inconsistencies, but we're talking about the non-TX case. Perhaps O
> could resubmit PUT(x) to the new P.
>
> I don't know how this behaves wrt rebalancing: are we flushing pending
> updates before installing the new CH?
>
> Thoughts?
>
>
>> I think that the source of optimization is that once primary decides to
>> backup the operation, he can forget about it and unlock the entry. So,
>> we don't need any ACK from primary unless it's an exception/noop
>> notification (as with conditional ops). If primary waited for ACK from
>> backup, we wouldn't save anything.
>
--
Radim Vansa <rvansa(a)redhat.com>
JBoss Performance Team