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From "David Smiley (JIRA)" <>
Subject [jira] [Commented] (SOLR-8744) Overseer operations need more fine grained mutual exclusion
Date Mon, 23 May 2016 14:29:13 GMT


David Smiley commented on SOLR-8744:

I looked over the patch just a little bit but the suggestion I'm about to give is mostly based
on the issue description & comments.  This is an interesting/fun programming problem.
 The main thing I don't like about the current design (and this is not a blocker, I'm not
vetoing) is that we obtain locks recursively on down to every object of lower depths (ultimately
to replicas).  Instead, I propose we consider an alternative design (to follow) in which we
only obtain a fixed small number of locks, which is for each parent.  This lowers the number
of locks, and also allows the actual lock nodes to be much more dynamic, being GC'ed when
the locks aren't in use.
* LockTree maintains a cache/map of string keys to weakly reference-able LockNode.  That's
right; they'll get GC'ed when not in use.  The root node will be strongly referenced apart
from the map so it's always there.  It's just a string key, with expectation we'll use some
separator to denote hierarchy.  When looking up a lock, we simply synchronize as obtaining
locks will be infrequent.  When looking up a Lock, it will recursively look up a parent, creating
it first if not there.  Getting the key is a simple matter of stringKey.substring(0, stringKey.lastIndexOf('/'))
and assuming all keys start with a leading "/" (thus the root key is the empty string).
* LockNode implements the JDK Lock interface, but only implementing {{lock()}} and {{unlock()}}.
 This makes the API easy for consumers -- it's a known abstraction for code using this utility.
* LockNode's fields consist of a parent LockNode and a JDK ReentrantReadWriteLock.  The ReentrantReadWriteLock
is instantiated with fairness=true.
* LockNode.lock: call parent.readLock() (see below) then call readWriteLock.writeLock().lock().
* LockNode.unlock: call readWriteLock.writeLock.unlock(); then parent.readUnlock(); (see below)
* LockNode.readLock (private method): call readLock on parent first (this is recursive), and
then after that call readWriteLock.readLock().lock().  Thus we lock from top-down (from root
down) in effect.
* LockNode.readUnlock (private method): call readWriteLock.readLock() then recursively call

What do you think?

> Overseer operations need more fine grained mutual exclusion
> -----------------------------------------------------------
>                 Key: SOLR-8744
>                 URL:
>             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. 
> SHARD(2)
> 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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