[infinispan-dev] New algorithm to handle remote commands

[Bela Ban]

On 18/09/14 13:03, Dan Berindei wrote:
> Thanks Pedro, this looks great.
>
> However, I don't think it's ok to treat CommitCommands/Pessimistic
> PrepareCommands as RemoteLockCommands just because they may send L1
> invalidation commands. It's true that those commands will block, but
> there's no need to wait for any other command before doing the L1
> invalidation. In fact, the non-tx writes on backup owners, which you
> consider to be non-blocking, can also send L1 invalidation commands (see
> L1NonTxInterceptor.invalidateL1).
>
> On the other hand, one of the good things that the remote executor did
> was to allow queueing lots of commands with a higher topology id, when
> one of the nodes receives the new topology much later than the others.
> We still have to consider each TopologyAffectedCommand as potentially
> blocking and put it through the remote executor.
>
> And InvalidateL1Commands are also TopologyAffectedCommands, so there's
> still a potential for deadlock when L1 is enabled and we have maxThreads
> write commands blocked sending L1 invalidations and those L1
> invalidation commands are stuck in the remote executor's queue on
> another node. And with (very) unlucky timing the remote executor might
> not even get to create maxThreads threads before the deadlock appears. I
> wonder if we could write a custom executor that checks what the first
> task in the queue is every second or so, and creates a bunch of new
> threads if the first task in the queue hasn't changed.
>
> You're right about the remote executor getting full as well, we're
> lacking any feedback mechanism to tell the sender to slow down, except
> for blocking the OOB thread.

JGroups sends credits back to the sender *after* the message has been 
delivered into the application. If the application is slow in processing 
the messages, or blocks for some time, then the sender will not receive 
enough credits and thus also slow down, or even block.

> I wonder if we could tell JGroups somehow
> to discard the message from inside MessageDispatcher.handle (e.g. throw
> a DiscardMessageException), so the sender has to retransmit it

At this point, JGroups considers the message *delivered* (as it has 
passed the UNICAST or NAKACK protocols), and it won't get resent. You 
cannot discard it either, as this will be a message loss. However, if 
you can tolerate loss, all is fine. E.g. if you discard a topo message 
with a lower ID, I don't think any harm is done in Infinispan. (?). To 
discard or not, is Infinispan's decision.
The other thing is to block, but this can have an impact back into the 
JGroups thread pools.

> and we
> don't block the OOB thread. That should allow us to set a size limit on
> the BlockingTaskAwareExecutor's blockedTasks collection as well. Bela, WDYT?
>
> Cheers
> Dan
>
>
> On Wed, Sep 17, 2014 at 7:17 PM, Pedro Ruivo <pedro at infinispan.org
> <mailto:pedro at infinispan.org>> wrote:
>
>     new link:
>     https://github.com/infinispan/infinispan/wiki/Remote-Command-Handler
>
>     On 09/17/2014 05:08 PM, Pedro Ruivo wrote:
>      > Hi,
>      >
>      > I've just wrote on the wiki a new algorithm to better handle the
>     remote
>      > commands. You can find it in [1].
>      >
>      > If you have questions, suggestion or just want to discuss some
>     aspect,
>      > please do in thread. I'll update the wiki page based on this
>     discussion
>      >
>      > Thanks.
>      >
>      > Cheers,
>      > Pedro
>      >
>      >
>     [1]https://github.com/infinispan/infinispan/wiki/Remote-Command-Handler-(Work-In-Progress...)
>      >
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-- 
Bela Ban, JGroups lead (http://www.jgroups.org)