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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-7263) Investigate more fine grained locking for checkAndPut/append/increment
Date Thu, 06 Dec 2012 23:39:21 GMT

    [ https://issues.apache.org/jira/browse/HBASE-7263?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13526009#comment-13526009

Lars Hofhansl commented on HBASE-7263:

You're talking heresy now :)

I have never questioned the need to rowlock, but since you ask directly... If we have MVCC
and your read-write locks, it seems we won't need the rowlocks any longer. They were only
really needed for the "atomic operation" such as increment/append and checkAndXXX.
> Investigate more fine grained locking for checkAndPut/append/increment
> ----------------------------------------------------------------------
>                 Key: HBASE-7263
>                 URL: https://issues.apache.org/jira/browse/HBASE-7263
>             Project: HBase
>          Issue Type: Improvement
>          Components: Transactions/MVCC
>            Reporter: Gregory Chanan
>            Assignee: Gregory Chanan
>            Priority: Minor
> HBASE-7051 lists 3 options for fixing an ACID-violation wrt checkAndPut:
> {quote}
> 1) Waiting for the MVCC to advance for read/updates: the downside is that you have to
wait for updates on other rows.
> 2) Have an MVCC per-row (table configuration): this avoids the unnecessary contention
of 1)
> 3) Transform the read/updates to write-only with rollup on read.. E.g. an increment would
just have the number of values to increment.
> {quote}
> HBASE-7051 and HBASE-4583 implement option #1.  The downside, as mentioned, is that you
have to wait for updates on other rows, since MVCC is per-row.
> Another option occurred to me that I think is worth investigating: rely on a row-level
read/write lock rather than MVCC.
> Here is pseudo-code for what exists today for read/updates like checkAndPut
> {code}
> (1)  Acquire RowLock
> (1a) BeginMVCC + Finish MVCC
> (2)  Begin MVCC
> (3)  Do work
> (4)  Release RowLock
> (5)  Append to WAL
> (6)  Finish MVCC
> {code}
> Write-only operations (e.g. puts) are the same, just without step 1a.
> Now, consider the following instead:
> {code}
> (1)  Acquire RowLock
> (1a) Grab+Release RowWriteLock (instead of BeginMVCC + Finish MVCC)
> (1b) Grab RowReadLock (new step!)
> (2)  Begin MVCC
> (3)  Do work
> (4)  Release RowLock
> (5)  Append to WAL
> (6)  Finish MVCC
> (7)  Release RowReadLock (new step!)
> {code}
> As before, write-only operations are the same, just without step 1a.
> The difference here is that writes grab a row-level read lock and hold it until the MVCC
is completed.  The nice property that this gives you is that read/updates can tell when the
MVCC is done on a per-row basis, because they can just try to acquire the write-lock which
will block until the MVCC is competed for that row in step 7.
> There is overhead for acquiring the read lock that I need to measure, but it should be
small, since there will never be any blocking on acquiring the row-level read lock.  This
is because the read lock can only block if someone else holds the write lock, but both the
write and read lock are only acquired under the row lock.
> I ran a quick test of this approach over a region (this directly interacts with HRegion,
so no client effects):
> - 30 threads
> - 5000 increments per thread
> - 30 columns per increment
> - Each increment uniformly distributed over 500,000 rows
> - 5 trials
> Better-Than-Theoretical-Max: (No locking or MVCC on step 1a): 10362.2 ms
> Today: 13950 ms
> The locking approach: 10877 ms
> So it looks like an improvement, at least wrt increment.  As mentioned, I need to measure
the overhead of acquiring the read lock for puts.

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