Hi, We are using Logback 1.1.3, and observed through jstack that quite a few threads were blocked in OutputStreamAppender.subAppend waiting to obtain the lock. This is a Play Framework app running in a dockerized environment, configured to log to STDOUT and papertrail. Stack trace: "New I/O worker #23" #75 prio=5 os_prio=0 tid=0x00007f6fc9764000 nid=0xaa waiting for monitor entry [0x00007f6edd3e7000] java.lang.Thread.State: BLOCKED (on object monitor) at ch.qos.logback.core.OutputStreamAppender.subAppend(OutputStreamAppender.java:211) - waiting to lock <0x00000000ab0f8f50> (a ch.qos.logback.core.spi.LogbackLock) at ch.qos.logback.core.OutputStreamAppender.append(OutputStreamAppender.java:103) at ch.qos.logback.core.UnsynchronizedAppenderBase.doAppend(UnsynchronizedAppenderBase.java:88) at ch.qos.logback.core.spi.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:48) at ch.qos.logback.classic.Logger.appendLoopOnAppenders(Logger.java:273) at ch.qos.logback.classic.Logger.callAppenders(Logger.java:260) at ch.qos.logback.classic.Logger.buildLoggingEventAndAppend(Logger.java:442) at ch.qos.logback.classic.Logger.filterAndLog_0_Or3Plus(Logger.java:396) at ch.qos.logback.classic.Logger.info(Logger.java:600) at play.api.LoggerLike$class.info(Logger.scala:93) at play.api.Logger$.info(Logger.scala:167) Any suggestions on what may be going on here? Thanks -greg