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https://issues.apache.org/jira/browse/SOLR-8744?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15323205#comment-15323205
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Scott Blum commented on SOLR-8744:
----------------------------------
Mostly LG. One completely minor comment:
{code}
+ if (blockedTasks.size() < MAX_BLOCKED_TASKS) {
+ blockedTasks.put(head.getId(), head);
+ }
{code}
Maybe unnecessary? It doesn't actually matter if blockedTasks.size() gets to
1100 items, the check at the top of the loop will keep it from growing
endlessly. If you drop these tasks on the floor, you'll just end up
re-fetching the bytes from ZK later.
> 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
> Priority: Blocker
> Labels: sharding, solrcloud
> Fix For: 6.1
>
> Attachments: SOLR-8744.patch, SOLR-8744.patch, SOLR-8744.patch,
> SOLR-8744.patch, SOLR-8744.patch, SOLR-8744.patch, SmileyLockTree.java,
> SmileyLockTree.java
>
>
> 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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