[infinispan-dev] DIST-SYNC, put(), a problem and a solution
Bela Ban
bban at redhat.com
Tue Jul 29 10:50:27 EDT 2014
On 29/07/14 16:42, Dan Berindei wrote:
> Have you tried regular optimistic/pessimistic transactions as well?
Yes, in my first impl. but since I'm making only 1 change per request, I
thought a TX is overkill.
> They *should* have less issues with the OOB thread pool than non-tx mode, and
> I'm quite curious how they stack against TO in such a large cluster.
Why would they have fewer issues with the thread pools ? AIUI, a TX
involves 2 RPCs (PREPARE-COMMIT/ROLLBACK) compared to one when not using
TXs. And we're sync anyway...
> On Tue, Jul 29, 2014 at 5:38 PM, Bela Ban <bban at redhat.com
> <mailto:bban at redhat.com>> wrote:
>
> Following up on my own email, I changed the config to use Pedro's
> excellent total order implementation:
>
> <transaction transactionMode="TRANSACTIONAL"
> transactionProtocol="TOTAL_ORDER" lockingMode="OPTIMISTIC"
> useEagerLocking="true" eagerLockSingleNode="true">
> <recovery enabled="false"/>
>
> With 100 nodes and 25 requester threads/node, I did NOT run into any
> locking issues !
>
> I could even go up to 200 requester threads/node and the perf was ~
> 7'000-8'000 requests/sec/node. Not too bad !
>
> This really validates the concept of lockless total-order dissemination
> of TXs; for the first time, this has been tested on a large(r) scale
> (previously only on 25 nodes) and IT WORKS ! :-)
>
> I still believe we should implement my suggested solution for non-TO
> configs, but short of configuring thread pools of 1000 threads or
> higher, I hope TO will allow me to finally test a 500 node Infinispan
> cluster !
>
>
> On 29/07/14 15:56, Bela Ban wrote:
> > Hi guys,
> >
> > sorry for the long post, but I do think I ran into an important
> problem
> > and we need to fix it ... :-)
> >
> > I've spent the last couple of days running the IspnPerfTest [1]
> perftest
> > on Google Compute Engine (GCE), and I've run into a problem with
> > Infinispan. It is a design problem and can be mitigated by sizing
> thread
> > pools correctly, but cannot be eliminated entirely.
> >
> >
> > Symptom:
> > --------
> > IspnPerfTest has every node in a cluster perform 20'000 requests
> on keys
> > in range [1..20000].
> >
> > 80% of the requests are reads and 20% writes.
> >
> > By default, we have 25 requester threads per node and 100 nodes in a
> > cluster, so a total of 2500 requester threads.
> >
> > The cache used is NON-TRANSACTIONAL / dist-sync / 2 owners:
> >
> > <namedCache name="clusteredCache">
> > <clustering mode="distribution">
> > <stateTransfer awaitInitialTransfer="true"/>
> > <hash numOwners="2"/>
> > <sync replTimeout="20000"/>
> > </clustering>
> >
> > <transaction transactionMode="NON_TRANSACTIONAL"
> > useEagerLocking="true"
> > eagerLockSingleNode="true" />
> > <locking lockAcquisitionTimeout="5000" concurrencyLevel="1000"
> > isolationLevel="READ_COMMITTED"
> useLockStriping="false" />
> > </namedCache>
> >
> > It has 2 owners, a lock acquisition timeout of 5s and a repl
> timeout of
> > 20s. Lock stripting is off, so we have 1 lock per key.
> >
> > When I run the test, I always get errors like those below:
> >
> > org.infinispan.util.concurrent.TimeoutException: Unable to
> acquire lock
> > after [10 seconds] on key [19386] for requestor
> [Thread[invoker-3,5,main]]!
> > Lock held by [Thread[OOB-194,ispn-perf-test,m5.1,5,main]]
> >
> > and
> >
> > org.infinispan.util.concurrent.TimeoutException: Node m8.1 timed out
> >
> >
> > Investigation:
> > ------------
> > When I looked at UNICAST3, I saw a lot of missing messages on the
> > receive side and unacked messages on the send side. This caused me to
> > look into the (mainly OOB) thread pools and - voila - maxed out !
> >
> > I learned from Pedro that the Infinispan internal thread pool (with a
> > default of 32 threads) can be configured, so I increased it to
> 300 and
> > increased the OOB pools as well.
> >
> > This mitigated the problem somewhat, but when I increased the
> requester
> > threads to 100, I had the same problem again. Apparently, the
> Infinispan
> > internal thread pool uses a rejection policy of "run" and thus
> uses the
> > JGroups (OOB) thread when exhausted.
> >
> > I learned (from Pedro and Mircea) that GETs and PUTs work as
> follows in
> > dist-sync / 2 owners:
> > - GETs are sent to the primary and backup owners and the first
> response
> > received is returned to the caller. No locks are acquired, so GETs
> > shouldn't cause problems.
> >
> > - A PUT(K) is sent to the primary owner of K
> > - The primary owner
> > (1) locks K
> > (2) updates the backup owner synchronously *while holding
> the lock*
> > (3) releases the lock
> >
> >
> > Hypothesis
> > ----------
> > (2) above is done while holding the lock. The sync update of the
> backup
> > owner is done with the lock held to guarantee that the primary and
> > backup owner of K have the same values for K.
> >
> > However, the sync update *inside the lock scope* slows things
> down (can
> > it also lead to deadlocks?); there's the risk that the request is
> > dropped due to a full incoming thread pool, or that the response
> is not
> > received because of the same, or that the locking at the backup owner
> > blocks for some time.
> >
> > If we have many threads modifying the same key, then we have a
> backlog
> > of locking work against that key. Say we have 100 requester
> threads and
> > a 100 node cluster. This means that we have 10'000 threads accessing
> > keys; with 2'000 writers there's a big chance that some writers
> pick the
> > same key at the same time.
> >
> > For example, if we have 100 threads accessing key K and it takes
> 3ms to
> > replicate K to the backup owner, then the last of the 100 threads
> waits
> > ~300ms before it gets a chance to lock K on the primary owner and
> > replicate it as well.
> >
> > Just a small hiccup in sending the PUT to the primary owner,
> sending the
> > modification to the backup owner, waitting for the response, or
> GC, and
> > the delay will quickly become bigger.
> >
> >
> > Verification
> > ----------
> > To verify the above, I set numOwners to 1. This means that the
> primary
> > owner of K does *not* send the modification to the backup owner,
> it only
> > locks K, modifies K and unlocks K again.
> >
> > I ran the IspnPerfTest again on 100 nodes, with 25 requesters, and NO
> > PROBLEM !
> >
> > I then increased the requesters to 100, 150 and 200 and the test
> > completed flawlessly ! Performance was around *40'000 requests
> per node
> > per sec* on 4-core boxes !
> >
> >
> > Root cause
> > ---------
> > *******************
> > The root cause is the sync RPC of K to the backup owner(s) of K while
> > the primary owner holds the lock for K.
> > *******************
> >
> > This causes a backlog of threads waiting for the lock and that
> backlog
> > can grow to exhaust the thread pools. First the Infinispan internal
> > thread pool, then the JGroups OOB thread pool. The latter causes
> > retransmissions to get dropped, which compounds the problem...
> >
> >
> > Goal
> > ----
> > The goal is to make sure that primary and backup owner(s) of K
> have the
> > same value for K.
> >
> > Simply sending the modification to the backup owner(s) asynchronously
> > won't guarantee this, as modification messages might get
> processed out
> > of order as they're OOB !
> >
> >
> > Suggested solution
> > ----------------
> > The modification RPC needs to be invoked *outside of the lock scope*:
> > - lock K
> > - modify K
> > - unlock K
> > - send modification to backup owner(s) // outside the lock scope
> >
> > The primary owner puts the modification of K into a queue from
> where a
> > separate thread/task removes it. The thread then invokes the
> PUT(K) on
> > the backup owner(s).
> >
> > The queue has the modified keys in FIFO order, so the modifications
> > arrive at the backup owner(s) in the right order.
> >
> > This requires that the way GET is implemented changes slightly:
> instead
> > of invoking a GET on all owners of K, we only invoke it on the
> primary
> > owner, then the next-in-line etc.
> >
> > The reason for this is that the backup owner(s) may not yet have
> > received the modification of K.
> >
> > This is a better impl anyway (we discussed this before) becuse it
> > generates less traffic; in the normal case, all but 1 GET
> requests are
> > unnecessary.
> >
> >
> >
> > Improvement
> > -----------
> > The above solution can be simplified and even made more efficient.
> > Re-using concepts from IRAC [2], we can simply store the modified
> *keys*
> > in the modification queue. The modification replication thread
> removes
> > the key, gets the current value and invokes a PUT/REMOVE on the
> backup
> > owner(s).
> >
> > Even better: a key is only ever added *once*, so if we have
> [5,2,17,3],
> > adding key 2 is a no-op because the processing of key 2 (in second
> > position in the queue) will fetch the up-to-date value anyway !
> >
> >
> > Misc
> > ----
> > - Could we possibly use total order to send the updates in TO ?
> TBD (Pedro?)
> >
> >
> > Thoughts ?
> >
> >
> > [1] https://github.com/belaban/IspnPerfTest
> > [2]
> >
> https://github.com/infinispan/infinispan/wiki/RAC:-Reliable-Asynchronous-Clustering
> >
>
> --
> Bela Ban, JGroups lead (http://www.jgroups.org)
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--
Bela Ban, JGroups lead (http://www.jgroups.org)
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