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https://issues.apache.org/jira/browse/DERBY-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Knut Anders Hatlen updated DERBY-2991:
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Attachment: d2991-preview-1e.diff
Updating the preview patch (1e). There are two changes from the previous
preview:
1) If the leaf page on which we're positioned is transformed into a branch page
while we don't hold the latch, we reposition by key. This fixes eight
regression test failures (ClassCastExceptions) seen with 1d. FWIW, derbyall and
suites.All now run without failures.
2) Copy the (possibly partial) key fetched in BTreeForwardScan when saving the
position, instead of re-reading the full key from the page. If the scan just
fetches a partial key, we fetch the missing parts only from the page.
This is not the same patch as I used in the latest performance test runs. That
patch copied the key as if the full key had been fetched by the scan. However,
since the tests only scan the index and don't go to the base table, the scans
don't fetch the RowLocation which is the last column in the index. So when we
save the position we still need to call Page.fetchFromSlot() to get the
RowLocation. So now the overhead with bulk fetch disabled is back at 1% (for
large key columns) to 10% (small/many key columns). I'm posting the patch for
reference anyway.
> Index split deadlock
> --------------------
>
> Key: DERBY-2991
> URL: https://issues.apache.org/jira/browse/DERBY-2991
> Project: Derby
> Issue Type: Bug
> Components: Store
> Affects Versions: 10.2.2.0, 10.3.1.4
> Environment: Windows XP, Java 6
> Reporter: Bogdan Calmac
> Assignee: Knut Anders Hatlen
> Attachments: d2991-preview-1a.diff, d2991-preview-1a.stat,
> d2991-preview-1b.diff, d2991-preview-1b.stat, d2991-preview-1c.diff,
> d2991-preview-1c.stat, d2991-preview-1d.diff, d2991-preview-1d.stat,
> d2991-preview-1e.diff, derby.log, InsertSelectDeadlock.java, perftest.diff,
> Repro2991.java, stacktraces_during_deadlock.txt
>
>
> After doing dome research on the mailing list, it appears that the index
> split deadlock is a known behaviour, so I will start by describing the
> theoretical problem first and then follow with the details of my test case.
> If you have concurrent select and insert transactions on the same table, the
> observed locking behaviour is as follows:
> - the select transaction acquires an S lock on the root block of the index
> and then waits for an S lock on some uncommitted row of the insert transaction
> - the insert transaction acquires X locks on the inserted records and if it
> needs to do an index split creates a sub-transaction that tries to acquire an
> X lock on the root block of the index
> In summary: INDEX LOCK followed by ROW LOCK + ROW LOCK followed by INDEX LOCK
> = deadlock
> In the case of my project this is an important issue (lack of concurrency
> after being forced to use table level locking) and I would like to contribute
> to the project and fix this issue (if possible). I was wondering if someone
> that knows the code can give me a few pointers on the implications of this
> issue:
> - Is this a limitation of the top-down algorithm used?
> - Would fixing it require to use a bottom up algorithm for better
> concurrency (which is certainly non trivial)?
> - Trying to break the circular locking above, I would first question why
> does the select transaction need to acquire (and hold) a lock on the root
> block of the index. Would it be possible to ensure the consistency of the
> select without locking the index?
> -----
> The attached test (InsertSelectDeadlock.java) tries to simulate a typical
> data collection application, it consists of:
> - an insert thread that inserts records in batch
> - a select thread that 'processes' the records inserted by the other thread:
> 'select * from table where id > ?'
> The derby log provides detail about the deadlock trace and
> stacktraces_during_deadlock.txt shows that the inser thread is doing an index
> split.
> The test was run on 10.2.2.0 and 10.3.1.4 with identical behaviour.
> Thanks,
> Bogdan Calmac.
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