我的服务器曾经在日志中看到APPARENT DEADLOCK。我有几台服务器在负载均衡器后运行，有趣的是我看到DEADLOCK同时出现在所有服务器上（有谁知道它为什么会影响所有服务器）？在此期间，通常需要200ms的MySQL查询需要> 60秒。这是日志的样子：
com.mchange.v2.async.ThreadPoolAsynchronousRunner: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@58780f76
-- APPARENT DEADLOCK!!! Complete Status:
Managed Threads: 3
Active Threads: 3
Active Tasks:
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@25ff87d4 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@10ccf7ef (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@3305ec37 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2)
Pending Tasks:
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@39cc9e5a
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@60d46f90
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@17509fea
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@b28bd63
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@56cbdc12
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@15a091b4
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@61ce325
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@48119520
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@4032fb7c
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@518eefff
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@30ea3b20
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@74960088
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@23a8fc7d
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@5ff0ee0
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@642d0644
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@207bc809
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@44d4936f
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@39a10d1b
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@3532334d
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@4bf79e62
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@2bd83398
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@1a202a2d
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@3eacda7f
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@495f5746
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@23f1f906
所以我来到Stack Overflow和found this answer which suggested我把statementCacheNumDeferredCloseThreads设置为1。我做到了这一点，我看到DEADLOCK不那么频繁，只在负载均衡器后面的几台服务器而不是全部。
日志现在看起来有点不同，但在DEADLOCK期间，查询仍然很长：
10 Oct 2018 06:33:32,037 [WARN] (Timer-0) com.mchange.v2.async.ThreadPoolAsynchronousRunner: com.mchange.v2.async.ThreadPoolAsynchronousRunner$DeadlockDetector@4f39ad63 -- APPARENT DEADLOCK!!! Complete Status:
Managed Threads: 3
Active Threads: 3
Active Tasks:
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@34dee200 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@3727ee6b (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask@4afb8b9 (com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0)
Pending Tasks:
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@384a3b5b
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@7bc700b0
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@731bfd15
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@a88e9bf
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@63f18b56
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@20f0c518
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@caf7746
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@41a7a27d
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@2ee32a24
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@81df2e5
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@7f7fa1e7
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@337503f
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@34b2f877
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@53dfbede
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@512d5ddb
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@68a25969
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@4bf0754a
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@65770ba4
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@5e0f4154
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@249c22ed
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@6c8e5911
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@3179550f
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@15d8a795
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@50966489
com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@4ecee95b
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StmtAcquireTask@35640ca0
com.mchange.v2.resourcepool.BasicResourcePool$AsyncTestIdleResourceTask@6550f196
com.mchange.v2.resourcepool.BasicResourcePool$AsyncTestIdleResourceTask@6816399
com.mchange.v2.resourcepool.BasicResourcePool$AsyncTestIdleResourceTask@3fbcd623
Pool thread stack traces:
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2,5,main]
com.mysql.jdbc.PreparedStatement.realClose(PreparedStatement.java:2765)
com.mysql.jdbc.StatementImpl.close(StatementImpl.java:541)
com.mchange.v1.db.sql.StatementUtils.attemptClose(StatementUtils.java:41)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask.run(GooGooStatementCache.java:404)
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#1,5,main]
com.mysql.jdbc.PreparedStatement.realClose(PreparedStatement.java:2765)
com.mysql.jdbc.StatementImpl.close(StatementImpl.java:541)
com.mchange.v1.db.sql.StatementUtils.attemptClose(StatementUtils.java:41)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask.run(GooGooStatementCache.java:404)
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#0,5,main]
com.mysql.jdbc.PreparedStatement.realClose(PreparedStatement.java:2765)
com.mysql.jdbc.StatementImpl.close(StatementImpl.java:541)
com.mchange.v1.db.sql.StatementUtils.attemptClose(StatementUtils.java:41)
com.mchange.v2.c3p0.stmt.GooGooStatementCache$1StatementCloseTask.run(GooGooStatementCache.java:404)
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
知道如何解决这个问题吗？我可以尝试完全禁用语句缓存，但我担心一般的性能损失。其他一些相关参数：
minPoolSize = 30
maxPoolSize = 30
maxStatements = 100
unreturnedConnectionTimeout = 500
idleConnectionTestPeriod = 60
acquireIncrements = 3
C3p0 version = 0.9.1.2
编辑：我忘了提及，在这个改进期间，我看到更少的死锁，我也增加了maxStatements，这可以解释改善。然而现在我只看到https://github.com/swaldman/c3p0/issues/53说version 0.9.2介绍了这个新参数statementCacheNumDeferredCloseThreads。我的版本太旧了。我没有关于此参数不存在的警告/错误。
也许现在为时已晚，但你是否试图增加numHelperThreads的数量？