Re: RFR: 8227269: Slow class loading when running JVM in debug mode

[serguei.spit...@oracle.com]

Hi Roman, Thank you for taking care about this scalability issue! I have a couple of quick comments. http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.04/src/jdk.jdwp.agent/share/native/libjdwp/classTrack.c.frames.html 72 /* 73 * Lock to protect deletedSignatureBag 74 */ 75 static jrawMonitorID deletedSignatureLock; 76 77 /* 78 * A bag containing all the deleted classes' signatures. Must be accessed under 79 * deletedTagLock, 80 */ 81 struct bag* deletedSignatureBag;   The comments contradict to each other.   I guess, the lock name at line 79 has to be deletedSignatureLock instead of deletedTagLock.   Also, comma at the end must be replaced with dot. 101 // Tag not found? Ignore. 102 if (klass == NULL) { 103 debugMonitorExit(deletedSignatureLock); 104 return; 105 } 106 107 // Scan linked-list. 108 jlong found_tag = klass->klass_tag; 109 while (klass != NULL && found_tag != tag) { 110 klass_ptr = &klass->next; 111 klass = *klass_ptr; 112 found_tag = klass->klass_tag; 113 } 114 115 // Tag not found? Ignore. 116 if (found_tag != tag) { 117 debugMonitorExit(deletedSignatureLock); 118 return; 119 }  The code above can be simplified, so that the lines 101-105 are not needed anymore.  It can be something like this: // Scan linked-list. while (klass != NULL && klass->klass_tag != tag) { klass_ptr = &klass->next; klass = *klass_ptr; } if (klass == NULL || klass->klass_tag != tag) { // klass not found - ignore. debugMonitorExit(deletedSignatureLock); return; } It will take more time when I get a chance to look at the rest. Thanks, Serguei On 12/21/19 13:24, Roman Kennke wrote: Here comes an update that resolves some races that happen when disconnecting an agent. In particular, we need to take the lock on basically every operation, and also need to check whether or not class-tracking is active and return an appropriate result (e.g. an empty list) when we're not. Updated webrev: http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.04/ Thanks, Roman So, here comes the O(1) implementation: - Whenever a class is 'prepared', it is registered with a tag, and we set-up a listener to get notified when it is unloaded. - Prepared classes are kept in a datastructure that is a table, which each entry being the head of a linked-list of KlassNode*. The table is indexed by tag % slot-count, and then simply prepend the new KlassNode*. This is O(1) operation. - When we get notified of unloading a class, we look up the signature of the reported tag in that table, and remember it in a bag. The KlassNode* is then unlinked from the table and deallocated. This is ~O(1) operation too, depending on the depth of the table. In my testcase which hammered the code with class-loads and unloads, I usually see depths of like 2-3, but not usually more. It should be ok. - when processUnloads() gets called, we simply hand out that bag, and allocate a new one. - I also added cleanup-code in classTrack_reset() to avoid leaking the signatures and KlassNode* etc when debug agent gets detached and/or re-attached (was missing before). - I also added locks around data-structure-manipulation (was missing before). - Also, I only activate this whole process when an actual listener gets registered on EI_GC_FINISH. This seems to happen right when attaching a jdb, not sure why jdb does that though. This may be something to improve in the future? In my tests, the performance of class-tracking itself looks really good. The bottleneck now is clearly actual synthesizing the class-unload events. I don't see how this can be helped when the debug agent asks for it? Updated webrev: http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.03/ Please let me know what you think of it. Thanks, Roman Alright, the perfectionist in me got me. I am implementing the even more efficient ~O(1) class tracking. Please hold off reviewing for now. Thanks,Roman Hi Chris, I'll have a look at this, although it might not be for a few days. In the meantime, maybe you can describe your new implementation in classTrack.c so it's easier to look through the changes. Sure. The purpose of this class-tracking is to be able to determine the signatures of unloaded classes when GC/class-unloading happened, so that we can generate the appropriate JDWP event. The current implementation does so by maintaining a table of currently prepared classes by building that table when classTrack is initialized, and then add new classes whenever a class gets loaded. When unloading occurs, that cache is rebuilt into a new table, and compared with the old table, and whatever is in the old, but not in the new table gets returned. The problem is that when GCs happen frequently and/or many classes get loaded+unloaded, this amounts to O(classCount*gcCount) complexity. The new implementation keeps a linked-list of prepared classes, and also tracks unloads via the listener cbTrackingObjectFree(). Whenever an unload/GC occurs, the list of prepared classes is scanned, and classes that are also in the deletedTagBag are unlinked (thus maintaining the prepared-classes-list) and its signature put in the list that gets returned. The implementation is not perfect. In order to determine whether or not a class is unloaded, it needs to scan the deletedTagBag. That process is therefore still O(unloadedClassCount). The assumption here is that unloadedClassCount << classCount. In my experiments this seems to be true, and also reasonable to expect. (I have some ideas how to improve the implementation to ~O(1) but it would be considerably more complex: have to maintain a (hash)table that maps tags -> KlassNode*, unlink them directly upon unload, and build the unloaded-signatures list there, but I don't currently see that it's worth the effort). In addition to all that, this process is only activated when there's an actual listener registered for EI_GC_FINISH. Thanks, Roman Chris On 12/18/19 5:05 AM, Roman Kennke wrote: Hello all, Issue: https://bugs.openjdk.java.net/browse/JDK-8227269 I am proposing what amounts to a rewrite of classTrack.c. It avoids throwing away the class cache on GC, and instead keeps track of loaded/unloaded classes one-by-one. In addition to that, it avoids this whole dance until an agent registers interest in EI_GC_FINISH. Webrev: http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.01/ Testing: manual testing of provided test scenarios and timing. Eg with the testcase provided here: https://bugzilla.redhat.com/show_bug.cgi?id=1751985 I am getting those numbers: unpatched: no debug: 84s with debug: 225s patched:   no debug: 85s with debug: 95s I also tested successfully through jdk/submit repo Can I please get a review? Thanks, Roman