I have one problem with performance in production because of, as I think, blocked threads . When I got thread dumps from production environment I was surprised that about 50% of thread are in status: *-- Blocked trying to get lock: ch/qos/logback/classic/sift/SiftingAppender@0x1111fe3e8[unlocked]* At the same time, as you can see, SiftingAppender is 'unlocked'. I have no idea why it's happened. If we deep into logback sources we will find there a synchronized method which is the 'thin' place I think: ch.qos.logback.core.AppenderBase.doAppend(E) public synchronized void doAppend(E eventObject) { // WARNING: The guard check MUST be the first statement in the // doAppend() method. // prevent re-entry. if (guard) { return; } ... } We use weblogic v.10.3.5.0 slf4j.version - 1.7.5 logback.version - 1.0.13 Has anybody ideas why it's happened and how to avoid this problem? It may be not logback's problem. I now trying to find resolution only.