[HDFS-2490] Upgradable lock to allow simutaleous read operation while reportDiff is in progress in processing block reports

• Type: Sub-task
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: namenode
• Assignee: Tomasz Nykiel
• Reporter: Tomasz Nykiel
• Created: 21/Oct/11 21:57
• Updated: 17/Sep/12 06:40

Description

Currently, FSNamesystem operations are protected by a single ReentrantReadWriteLock, which allows for having multiple concurrent readers to perform reads, and a single writer to perform writes. There are, however, operations whose execution has primarily reading nature, but occasionally they write.
The finest example is processing block reports - currently the entire processing is done under writeLock(). With HDFS-395 (explicit deletion acks), processing a block report is primarily a read operation (reportDiff()) after which only very few blocks need to be updated. In fact, we noticed this number to be very low, or even zero blocks.

It would be desirable to have an upgradeable read lock, which would allow for performing other reads during the first "read" part of reportDiff() (and possibly other operations.

We implemented such mechanism, which provides writeLock(), readLock(), upgradeableReadLock, upgradeLock(), and downgradeLock(). I achieved this be emloying two ReentrantReadWriteLock's - one protects writes (lock1), the other one reads (lock2).

Hence, we have:

writeLock()
lock1.writeLock().lock()
lock2.writeLock().lock()

readLock()
lock2.readLock().lock()

upgradeableReadLock()
lock1.writeLock().lock()

upgrade()
lock2.writeLock().lock()

--------------------------

Hence a writeLock() is essentially equivalent to upgradeableLock()+upgrade()

• two writeLocks are mutually exclusive because of lock1.writeLock
• a writeLock and upgradeableLock are mutually exclusive as above
• readLock is mutually exclusive with upgradeableLock()+upgrade() OR writeLock because of lock2.writeLock
• readLock() + writeLock() causes a deadlock, the same as currently
• writeLock() + readLock() does not cause deadlocks

--------------------------

I am conviced to the soundness of this mechanism.
The overhead comes from having two locks, and in particular, writes need to acquire both of them.
We deployed this feature, we used the upgradeableLock() ONLY for processing reports.
Our initial, but not exhaustive experiments have shown that it had a very detrimental effect on the NN throughput - writes were taking up to twice as long.
This is very unexpected, and hard to explain by only the overhead of acquiring additional lock for writes.

I would like to ask for input, as maybe I am missing some fundamental problem here.
I am attaching a java class which implements this locking mechanism.