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https://issues.apache.org/jira/browse/SOLR-8744?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15297086#comment-15297086
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David Smiley commented on SOLR-8744:
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The main purpose of my suggestion is simplicity: LockTree (public) and
internally LockTree.Node (private) -- that's it; no new abstractions, and it'd
have very little code. Node would keep track of it's parent only, not
children. No Session or LockObject. Returning a JDK Lock helps in that regard
(it is an existing abstraction and if we wanted to it'd be easy to support all
its methods) but it's not critical -- my suggested LockTree.Node could be
public and we don't implement Lock if you feel that's better. I disagree with
you on the semantics being different vs the same but whatever. It just so
happens that my suggestion might yield something general purpose but that's not
my objective, it's simplicity. Locking on fewer objects is nice too, but
perhaps it won't become a real issue with your proposal. Perhaps I'm not fully
appreciating the task at hand -- I admit I glossed over the starvation issue
and the code here concerned with marking busy/not. I hoped the "fairness"
boolean configuration to JDK's ReentrantReadWriteLock would address that but
maybe not?
bq. (me) Getting the key is a simple matter of stringKey.substring(0,
stringKey.lastIndexOf('/'))
Just a minor implementation detail to get a parent key instead of working with
a List<String> -- nevermind.
> Overseer operations need more fine grained mutual exclusion
> -----------------------------------------------------------
>
> Key: SOLR-8744
> URL: https://issues.apache.org/jira/browse/SOLR-8744
> Project: Solr
> Issue Type: Improvement
> Components: SolrCloud
> Affects Versions: 5.4.1
> Reporter: Scott Blum
> Assignee: Noble Paul
> Labels: sharding, solrcloud
> Attachments: SOLR-8744.patch
>
>
> SplitShard creates a mutex over the whole collection, but, in practice, this
> is a big scaling problem. Multiple split shard operations could happen at
> the time time, as long as different shards are being split. In practice,
> those shards often reside on different machines, so there's no I/O bottleneck
> in those cases, just the mutex in Overseer forcing the operations to be done
> serially.
> Given that a single split can take many minutes on a large collection, this
> is a bottleneck at scale.
> Here is the proposed new design
> There are various Collection operations performed at Overseer. They may need
> exclusive access at various levels. Each operation must define the Access
> level at which the access is required. Access level is an enum.
> CLUSTER(0)
> COLLECTION(1)
> SHARD(2)
> REPLICA(3)
> The Overseer node maintains a tree of these locks. The lock tree would look
> as follows. The tree can be created lazily as and when tasks come up.
> {code}
> Legend:
> C1, C2 -> Collections
> S1, S2 -> Shards
> R1,R2,R3,R4 -> Replicas
> Cluster
> / \
> / \
> C1 C2
> / \ / \
> / \ / \
> S1 S2 S1 S2
> R1, R2 R3.R4 R1,R2 R3,R4
> {code}
> When the overseer receives a message, it tries to acquire the appropriate
> lock from the tree. For example, if an operation needs a lock at a Collection
> level and it needs to operate on Collection C1, the node C1 and all child
> nodes of C1 must be free.
> h2.Lock acquiring logic
> Each operation would start from the root of the tree (Level 0 -> Cluster) and
> start moving down depending upon the operation. After it reaches the right
> node, it checks if all the children are free from a lock. If it fails to
> acquire a lock, it remains in the work queue. A scheduler thread waits for
> notification from the current set of tasks . Every task would do a
> {{notify()}} on the monitor of the scheduler thread. The thread would start
> from the head of the queue and check all tasks to see if that task is able to
> acquire the right lock. If yes, it is executed, if not, the task is left in
> the work queue.
> When a new task arrives in the work queue, the schedulerthread wakes and just
> try to schedule that task.
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