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Joseph Lynch edited comment on CASSANDRA-14922 at 12/13/18 3:51 AM: -------------------------------------------------------------------- I found a better workaround (in [bfd8c328|https://github.com/apache/cassandra/commit/bfd8c328f92e6bd2c82269a048f6b868179f484a]) for the jna {{Native}} Classloader leak is to just call the [register|https://github.com/java-native-access/jna/blob/365f6b343a92427e7890cae0c16df7b6c4c254d4/src/com/sun/jna/Native.java#L1446] API that doesn't add the {{InstanceClassLoader}} to a global static map in {{Native}}. This still loads the libraries and doesn't leak the ClassLoader into a global static map. If we're concerned about the changes to the {{NativeLibrary}} I can pursue other option. I also found another thread that leaks only during ant test runs which is fixed in [512d15b5f|https://github.com/apache/cassandra/commit/512d15b5f08bda1b0dc8a952ce950b5c4341f992]. Furthermore I believe I know why the JVM was still not releasing the classloaders even after they are no longer referenced: [SoftReferences|https://docs.oracle.com/javase/8/docs/api/java/lang/ref/SoftReference.html] are apparently just ... not collected. It looks like Metaspace just grows without bound until we run the machine OOM (not the JVM, the machine). I tried the following after eliminating all strong references: 1. No JVM tuning. This _did not_ work as the metaspace would grow without bound and eventually OOM the machine (not the JVM, the machine). 2. {{-XX:SoftRefLRUPolicyMSPerMB=0}} and or {{-XX:MaxMetaspaceSize=256M}}. This *worked* and caused the ClassLoaders to be released. With MaxMetaspaceSize we also successfully limit the offheap Metaspace which is nice. 3. {{-XX:MaxMetaspaceSize=256M}} and or {{-XX:MetaspaceSize=100M}}. This _did not_ work. Setting the Max size would cause an {{java.lang.OutOfMemoryError: Metaspace}}, setting just the size didn't do anything (machine would just OOM itself anyways) 4. {{-XX:UseConcMarkSweepGC}} and or {{-XX:CMSClassUnloadingEnabled}}. This _did not_ work. Again the metaspace just grew without bound and ran out of memory. I think that running all unit tests under #2 is a pretty reasonable thing to do (we shouldn't be leaking Metaspace imo). I've pursued that option in [86f982b1|https://github.com/apache/cassandra/commit/86f982b19ba23fc5efcd7421449af4a84d93711b] and it appears to work looking at my [profiler|https://issues.apache.org/jira/secure/attachment/12951598/Metaspace_Actually_Collected.png]. [~ifesdjeen]/[~benedict] Do you guys think it's acceptable to run all unit tests with the off-heap Metaspace limited to 256 megabytes and {{SoftRefLRUPolicyMSPerMB}} set to 0 to tell the GC to actually collect soft references? It's different than the status quo but I think we probably shouldn't leak metaspace (aka permgen from java7/6) and we probably shouldn't rely on soft references for things to work generally... I checked and both options are available in openjdk8+, but I'm not sure if there is a better way. Other than the ThreadLocal reflection hacks I think the branch is almost ready to squash and merge, what do you think? was (Author: jolynch): I found a better workaround (in [bfd8c328|https://github.com/apache/cassandra/commit/bfd8c328f92e6bd2c82269a048f6b868179f484a]) for the jna {{Native}} Classloader leak is to just call the [register|https://github.com/java-native-access/jna/blob/365f6b343a92427e7890cae0c16df7b6c4c254d4/src/com/sun/jna/Native.java#L1446] API that doesn't add the {{InstanceClassLoader}} to a global static map in {{Native}}. This still loads the libraries and doesn't leak the ClassLoader into a global static map. If we're concerned about the changes to the {{NativeLibrary}} I can pursue other option. I also found another thread that leaks only during ant test runs which is fixed in [512d15b5f|https://github.com/apache/cassandra/commit/512d15b5f08bda1b0dc8a952ce950b5c4341f992]. Furthermore I believe I know why the JVM was still not releasing the classloaders even after they are no longer referenced: [SoftReferences|https://docs.oracle.com/javase/8/docs/api/java/lang/ref/SoftReference.html] are apparently just ... not collected. It looks like Metaspace just grows without bound until we run the machine OOM (not the JVM, the machine). I tried the following after eliminating all strong references: 1. No JVM tuning. This _did not_ work as the metaspace would grow without bound and eventually OOM the machine (not the JVM, the machine). 2. {{-XX:SoftRefLRUPolicyMSPerMB=0}} and or {{-XX:MaxMetaspaceSize=256M}}. This *worked* and caused the ClassLoaders to be released. With MaxMetaspaceSize we also successfully limit the neww 3. {{-XX:MaxMetaspaceSize=256M}} and or {{-XX:MetaspaceSize=100M}}. This _did not_ work. Setting the Max size would cause an {{java.lang.OutOfMemoryError: Metaspace}}, setting just the size didn't do anything (machine would just OOM itself anyways) 4. {{-XX:UseConcMarkSweepGC}} and or {{-XX:CMSClassUnloadingEnabled}}. This _did not_ work. Again the metaspace just grew without bound and ran out of memory. I think that running all unit tests under #2 is a pretty reasonable thing to do (we shouldn't be leaking Metaspace imo). I've pursued that option in [86f982b1|https://github.com/apache/cassandra/commit/86f982b19ba23fc5efcd7421449af4a84d93711b] and it appears to work looking at my [profiler|https://issues.apache.org/jira/secure/attachment/12951598/Metaspace_Actually_Collected.png]. [~ifesdjeen]/[~benedict] Do you guys think it's acceptable to run all unit tests with the off-heap Metaspace limited to 256 megabytes and {{SoftRefLRUPolicyMSPerMB}} set to 0 to tell the GC to actually collect soft references? It's different than the status quo but I think we probably shouldn't leak metaspace (aka permgen from java7/6) and we probably shouldn't rely on soft references for things to work generally... I checked and both options are available in openjdk8+, but I'm not sure if there is a better way. Other than the ThreadLocal reflection hacks I think the branch is almost ready to squash and merge, what do you think? > In JVM dtests need to clean up after instance shutdown > ------------------------------------------------------ > > Key: CASSANDRA-14922 > URL: https://issues.apache.org/jira/browse/CASSANDRA-14922 > Project: Cassandra > Issue Type: Bug > Components: Testing > Reporter: Joseph Lynch > Assignee: Joseph Lynch > Priority: Minor > Attachments: AllThreadsStopped.png, ClassLoadersRetaining.png, > Leaking_Metrics_On_Shutdown.png, MainClassRetaining.png, > Metaspace_Actually_Collected.png, OnlyThreeRootsLeft.png, > no_more_references.png > > > Currently the unit tests are failing on circleci ([example > one|https://circleci.com/gh/jolynch/cassandra/300#tests/containers/1], > [example > two|https://circleci.com/gh/rustyrazorblade/cassandra/44#tests/containers/1]) > because we use a small container (medium) for unit tests by default and the > in JVM dtests are leaking a few hundred megabytes of memory per test right > now. This is not a big deal because the dtest runs with the larger containers > continue to function fine as well as local testing as the number of in JVM > dtests is not yet high enough to cause a problem with more than 2GB of > available heap. However we should fix the memory leak so that going forwards > we can add more in JVM dtests without worry. > I've been working with [~ifesdjeen] to debug, and the issue appears to be > unreleased Table/Keyspace metrics (screenshot showing the leak attached). I > believe that we have a few potential issues that are leading to the leaks: > 1. The > [{{Instance::shutdown}}|https://github.com/apache/cassandra/blob/f22fec927de7ac291266660c2f34de5b8cc1c695/test/distributed/org/apache/cassandra/distributed/Instance.java#L328-L354] > method is not successfully cleaning up all the metrics created by the > {{CassandraMetricsRegistry}} > 2. The > [{{TestCluster::close}}|https://github.com/apache/cassandra/blob/f22fec927de7ac291266660c2f34de5b8cc1c695/test/distributed/org/apache/cassandra/distributed/TestCluster.java#L283] > method is not waiting for all the instances to finish shutting down and > cleaning up before continuing on > 3. I'm not sure if this is an issue assuming we clear all metrics, but > [{{TableMetrics::release}}|https://github.com/apache/cassandra/blob/4ae229f5cd270c2b43475b3f752a7b228de260ea/src/java/org/apache/cassandra/metrics/TableMetrics.java#L951] > does not release all the metric references (which could leak them) > I am working on a patch which shuts down everything and assures that we do > not leak memory. -- This message was sent by Atlassian JIRA (v7.6.3#76005) --------------------------------------------------------------------- To unsubscribe, e-mail: commits-unsubscr...@cassandra.apache.org For additional commands, e-mail: commits-h...@cassandra.apache.org