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https://issues.apache.org/jira/browse/HIVE-15077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16374883#comment-16374883
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Alan Gates commented on HIVE-15077:
-----------------------------------

+1.  The previous code was trying to save time by only comparing against some 
of the locks, but since the sort or was changed in HIVE-10242 that no longer 
works.  

> Acid LockManager is unfair
> --------------------------
>
>                 Key: HIVE-15077
>                 URL: https://issues.apache.org/jira/browse/HIVE-15077
>             Project: Hive
>          Issue Type: Bug
>          Components: Transactions
>    Affects Versions: 2.3.0
>            Reporter: Eugene Koifman
>            Assignee: Eugene Koifman
>            Priority: Blocker
>         Attachments: HIVE-15077.02.patch
>
>
> HIVE-10242 made the acid LM unfair.
> In TxnHandler.checkLock(), suppose we are trying to acquire SR5  (the number 
> is extLockId).  
> Then 
> LockInfo[] locks = lockSet.toArray(new LockInfo[lockSet.size()]);
> may look like this (all explicitly listed locks are in Waiting state)
> {...., SR5 SW3 X4}
> So the algorithm will find SR5 in the list and start looking backwards (to 
> the left).
> According to IDs, SR5 should wait for X4 to be granted but X4 won't even be 
> examined and so SR5 may be granted.
> Theoretically, this could cause starvation.
> The query that generates the list already has
> query.append(" and hl_lock_ext_id <= ").append(extLockId);
> but it should use "<" rather than "<=" to exclude the locks being checked 
> from "locks" list which will make the algorithm look at all locks "in front" 
> of a given lock.
> Here is an example (add to TestDbTxnManager2)
> {noformat}
>   @Test
>   public void testFairness2() throws Exception {
>     dropTable(new String[]{"T7"});
>     CommandProcessorResponse cpr = driver.run("create table if not exists T7 
> (a int) partitioned by (p int) stored as orc TBLPROPERTIES 
> ('transactional'='true')");
>     checkCmdOnDriver(cpr);
>     checkCmdOnDriver(driver.run("insert into T7 partition(p) 
> values(1,1),(1,2)"));//create 2 partitions
>     cpr = driver.compileAndRespond("select a from T7 ");
>     checkCmdOnDriver(cpr);
>     txnMgr.acquireLocks(driver.getPlan(), ctx, "Fifer");//gets S lock on T7
>     HiveTxnManager txnMgr2 = 
> TxnManagerFactory.getTxnManagerFactory().getTxnManager(conf);
>     swapTxnManager(txnMgr2);
>     cpr = driver.compileAndRespond("alter table T7 drop partition (p=1)");
>     checkCmdOnDriver(cpr);
>     //tries to get X lock on T7.p=1 and gets Waiting state
>     LockState lockState = ((DbTxnManager) 
> txnMgr2).acquireLocks(driver.getPlan(), ctx, "Fiddler", false);
>     List<ShowLocksResponseElement> locks = getLocks();
>     Assert.assertEquals("Unexpected lock count", 4, locks.size());
>     checkLock(LockType.SHARED_READ, LockState.ACQUIRED, "default", "T7", 
> null, locks);
>     checkLock(LockType.SHARED_READ, LockState.ACQUIRED, "default", "T7", 
> "p=1", locks);
>     checkLock(LockType.SHARED_READ, LockState.ACQUIRED, "default", "T7", 
> "p=2", locks);
>     checkLock(LockType.EXCLUSIVE, LockState.WAITING, "default", "T7", "p=1", 
> locks);
>     HiveTxnManager txnMgr3 = 
> TxnManagerFactory.getTxnManagerFactory().getTxnManager(conf);
>     swapTxnManager(txnMgr3);
>     //this should block behind the X lock on  T7.p=1
>     cpr = driver.compileAndRespond("select a from T7");
>     checkCmdOnDriver(cpr);
>     txnMgr3.acquireLocks(driver.getPlan(), ctx, "Fifer");//gets S lock on T6
>     locks = getLocks();
>     Assert.assertEquals("Unexpected lock count", 7, locks.size());
>     checkLock(LockType.SHARED_READ, LockState.ACQUIRED, "default", "T7", 
> null, locks);
>     checkLock(LockType.SHARED_READ, LockState.ACQUIRED, "default", "T7", 
> "p=1", locks);
>     checkLock(LockType.SHARED_READ, LockState.ACQUIRED, "default", "T7", 
> "p=2", locks);
>     checkLock(LockType.SHARED_READ, LockState.ACQUIRED, "default", "T7", 
> null, locks);
>     checkLock(LockType.SHARED_READ, LockState.ACQUIRED, "default", "T7", 
> "p=1", locks);
>     checkLock(LockType.SHARED_READ, LockState.ACQUIRED, "default", "T7", 
> "p=2", locks);
>     checkLock(LockType.EXCLUSIVE, LockState.WAITING, "default", "T7", "p=1", 
> locks);
>   }
> {noformat}
> The 2nd {{locks = getLocks();}} output shows that all locks for the 2nd 
> {{select * from T7}} are all acquired while they should block behind the X 
> lock to be fair.



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