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From "Gregory Chanan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-7263) Investigate more fine grained locking for checkAndPut/append/increment
Date Tue, 04 Dec 2012 02:25:58 GMT

    [ https://issues.apache.org/jira/browse/HBASE-7263?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13509440#comment-13509440
] 

Gregory Chanan commented on HBASE-7263:
---------------------------------------

bq. Do these new locks time out like original RowLock?
Not yet, the write-lock should though.

bq. This has the potential that Puts starve out Increments, right? (not a big deal, though)
I don't think so.  Consider some set of puts that are holding the readlock, but not the rowlock.
 The next operation to go will be whoever grabs the rowlock next (since it is grabbed before
the read or write lock), which is exactly the same as today.

bq. Also, I'd be a bit skeptical about another mechanism for change visibility.
Well, I also investigated making MVCC more aware (either making it totally per row or having
it track the rows being modified and providing a "waitForRowComplete" method, but those were
way too complicated :).  That's not an argument for adding another mechanism, though.  I agree
we should only do this if the performance is worth it.

bq. Do the result even get better if you spread the increments out over fewer rows?
I'm not sure.  My thinking was to spread the increments over a large number of rows so that
the contention is on MVCC, not on the row lock.  I thought the "Today" number for the increment
test would be way worse, actually, because you'd essentially get only one increment at a time
actually doing work.  I need to investigate further.  It's possible that multiple increments
are doing work at the same time, though; any number can technically finish completeMemstoreInsert(beginMemstoreInsert)
before another starts a new transaction with beginMemstoreInsert.
                
> 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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