Re: Slow class loading when running JVM in debug mode

[Chris Plummer]

With respect to the specific issue brought up in https://youtrack.jetbrains.com/issue/JBR-1611: "If we look into the code, we'll see that whenever a new class is loaded and an event about it is delivered, when a garbage collection has occurred, classTrack_processUnloads iterates over all loaded classes to see if any of them have been unloaded. This leads to O(classCount * gcCount) performance, which in case of frequent GCs (and they are frequent, especially the minor ones) is close to O(classCount^2). In IDEA, we have quite a lot of classes, especially counting all lambdas, so this results in quite significant overhead." The debug agent calls JVMTI GetLoadedClasses() during initialization to get a cache of all prepared classes. It keeps that cache up-to-date by getting JVMTI CLASS_PREPARE events. When there is a gc, the debug agent basically throws away the cache and creates a new one by calling GetLoadedClasses() again. It also compares the old and new caches to determine which classes where unloaded, and generates JDWP CLASS_UNLOAD events for them (if there is a debugger attached that wants them). It might be possible to defer initialization of the loaded classes cache (and any maintenance of it) until there is a debugger attached. I'm not sure if the only reason for the cache is for delivery of CLASS_UNLOAD events, but at first glance that appears to be the case. Chris On 6/20/19 1:31 PM, Volker Simonis wrote: Hi Egor, yes, I‘d say this is something well known. The reason for this is that a debugging agent will request some JVMTI capabilities like "can_access_local_variables" which can only be requested at JVM startup. However, once these capabilities have been taken, certain kinds of optimizations like for example Escape Analysis, can’t be done anymore which results in a performance degradation even if you don’t ever attach a debugger. We‘ve always enabled debugging in our commercial SAP JVM and called it „Debugging on Demand“ but usually didn’t observed any performance slowdown of more than 3-5% at most. We’ve started contributing some of the improvements we’ve done to make this possible to the OpenJDK. I’ve not looked into your concrete class loading use case. Maybe it’s possible to improve that. First you have to check by which JVMTI capability it is triggered and if that capability is really necessary for debugging and can only be requested at startup. If the answer is yes, you can still check if it’s not possible to improve the implementation of that capability in order to get a better runtime behavior. Regards, Volker Egor Ushakov <egor.usha...@jetbrains.com> schrieb am Do. 20. Juni 2019 um 11:36: Hi everyone! we have detected that a process started with the debug agent (even when debugger is not actually attached) may run significantly slower than without the agent, see: https://youtrack.jetbrains.com/issue/JBR-1611 We all thought that without the debugger attached there should not be a difference. Is that something well known? Should we file a bug? Thanks, Egor -- Egor Ushakov Software Developer JetBrains http://www.jetbrains.com The Drive to Develop