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From "Hal Mo (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2490) Upgradable lock to allow simutaleous read operation while reportDiff is in progress in processing block reports
Date Mon, 17 Sep 2012 06:41:07 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2490?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13456792#comment-13456792

Hal Mo commented on HDFS-2490:

Hi, Tomasz Nykiel. 
I also tried to do the same thing.
I asked this question on mail list, you can join the discussion.


Hal Mo
> Upgradable lock to allow simutaleous read operation while reportDiff is in progress in
processing block reports
> ---------------------------------------------------------------------------------------------------------------
>                 Key: HDFS-2490
>                 URL: https://issues.apache.org/jira/browse/HDFS-2490
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: name-node
>            Reporter: Tomasz Nykiel
>            Assignee: Tomasz Nykiel
>         Attachments: FSNamesystemLock.java
> 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.

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