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  1. Hive
  2. HIVE-24569

LLAP daemon leaks file descriptors/log4j appenders

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      With HIVE-9756 query logs in LLAP are directed to different files (file per query) using a Log4j2 routing appender. Without a purge policy in place, appenders are created dynamically by the routing appender, one for each query, and remain in memory forever. The dynamic appenders write to files so each appender holds to a file descriptor.

      Further work HIVE-14224 has mitigated the issue by introducing a custom purging policy (LlapRoutingAppenderPurgePolicy) which deletes the dynamic appenders (and closes the respective files) when the query is completed (org.apache.hadoop.hive.llap.daemon.impl.QueryTracker#handleLogOnQueryCompletion).

      However, in the presence of multiple threads appending to the logs there are race conditions. In an internal Hive cluster the number of file descriptors started going up approx one descriptor leaking per query. After some debugging it turns out that one thread (running the QueryTracker#handleLogOnQueryCompletion) signals that the query has finished and thus the purge policy should get rid of the respective appender (and close the file) while another (Task-Executor-0) attempts to append another log message for the same query. The initial appender is closed after the request from the query tracker but a new one is created to accomodate the message from the task executor and the latter is never removed thus creating a leak.

      Similar leaks have been identified and fixed for HS2 with the most similar one being that described here.

      The problem relies on the timing of threads so it may not manifestate in all versions between 2.2.0 and 4.0.0. Usually the leak can be seen either via lsof (or other similar command) with the following output:

      # 1494391 is the PID of the LLAP daemon process
      ls -ltr /proc/1494391/fd
      lrwx------ 1 hive hadoop 64 Dec 24 12:08 978 -> /hadoop/yarn/log/application_1608659125567_0006/container_e04_1608659125567_0006_01_000002/hive_20201224121724_66ce273d-54a9-4dcd-a9fb-20cb5691cef7-dag_1608659125567_0008_194.log
      lrwx------ 1 hive hadoop 64 Dec 24 12:08 977 -> /hadoop/yarn/log/application_1608659125567_0006/container_e04_1608659125567_0006_01_000002/hive_20201224121804_ce53eeb5-c73f-4999-b7a4-b4dd04d4e4de-dag_1608659125567_0008_197.log
      lrwx------ 1 hive hadoop 64 Dec 24 12:08 974 -> /hadoop/yarn/log/application_1608659125567_0006/container_e04_1608659125567_0006_01_000002/hive_20201224122002_1693bd7d-2f0e-4673-a8d1-b7cb14a02204-dag_1608659125567_0008_204.log
      lrwx------ 1 hive hadoop 64 Dec 24 12:08 989 -> /hadoop/yarn/log/application_1608659125567_0006/container_e04_1608659125567_0006_01_000002/hive_20201224121909_6a56218f-06c7-4906-9907-4b6dd824b100-dag_1608659125567_0008_201.log
      lrwx------ 1 hive hadoop 64 Dec 24 12:08 984 -> /hadoop/yarn/log/application_1608659125567_0006/container_e04_1608659125567_0006_01_000002/hive_20201224121754_78ef49a0-bc23-478f-9a16-87fa25e7a287-dag_1608659125567_0008_196.log
      lrwx------ 1 hive hadoop 64 Dec 24 12:08 983 -> /hadoop/yarn/log/application_1608659125567_0006/container_e04_1608659125567_0006_01_000002/hive_20201224121855_e65b9ebf-b2ec-4159-9570-1904442b7048-dag_1608659125567_0008_200.log
      lrwx------ 1 hive hadoop 64 Dec 24 12:08 981 -> /hadoop/yarn/log/application_1608659125567_0006/container_e04_1608659125567_0006_01_000002/hive_20201224121818_e9051ae3-1316-46af-aabb-22c53ed2fda7-dag_1608659125567_0008_198.log
      lrwx------ 1 hive hadoop 64 Dec 24 12:08 980 -> /hadoop/yarn/log/application_1608659125567_0006/container_e04_1608659125567_0006_01_000002/hive_20201224121744_fcf37921-4351-4368-95ee-b5be2592d89a-dag_1608659125567_0008_195.log
      lrwx------ 1 hive hadoop 64 Dec 24 12:08 979 -> /hadoop/yarn/log/application_1608659125567_0006/container_e04_1608659125567_0006_01_000002/hive_20201224121837_e80c0024-f6bc-4b3c-85ed-5c0c85c55787-dag_1608659125567_0008_199.log

      or in the heap dump with many appenders (in my case LlapWrappedAppender) holding indirectly open file descriptors:


        1. llap-appender-gc-roots.png
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          Stamatis Zampetakis

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