java memory usage investigation

src: Il faut sauver le dernier giga de RAM - 2024-12-05

Abstract

Some tips to investigate memory usage:

• generate a memory dump:
  • jmap -dump:[live],format=b,file=<file_path> <pid>
  • jcmd <pid> GC.heap_dump <file_path>
• use Eclipse Memory Analyzer to read the memory dump file
• use -Xmx with small value and let your app crash locally
• use -XX:+HeapDumpOnOutOfMemoryError
• use XX:StartFlightRecording=duration=200s which generates a JFR file, readable using JDK missing control
• use pg_stat_statements extension:

# add to your postgresql.conf
shared_preload_libraries = 'pg_stat_statements'

CREATE EXTENSION pg_stat_statements;
-- don't forget to reset if you are debugging to have fresh stats
SELECT pg_stat_statements_reset();

• use closed and opened projections by creating custom DTOs used as returned type in your JPA interfaces:

interface PersonRepository {
  // closed projections
  String getFirstName();
  String getLastName();
  PersonTrait getPersonTrait();
 
  record PersonFullName(String firstName, String lastName, Gender gender) {}
 
  // opened projections
  default String getFullname() {
    return getFirstName().concat(" ").concat(getLastName());
  }
}

• get bulk of items:

@Repository
public interface PersonRepository extends JpaRepository<Person, UUID> {
  Stream<Person> getByFirstName(String firstName);
  @QueryHints(value = {
    @QueryHint(name = HINT_FETCH_SIZE, value = "300000")
  })
  Stream<Person> getByFirstName(String firstName);
  Page<Person> getByFirstName(String firstName, Pageable p);
  Slice<Person> getByFirstName(String firstName, Pageable p);
}

• use hypersistence-utils to detect where the hibernate SQL query was executed (don’t forget to set the log level logging.level.io.hypersistence.utils.hybernate.query=DEBUG)

<dependency>
  <groupId>io.hypersistence</groupId>
  <artifactId>hypersistence-utils-hibernate-63</artifactId>
  <version>3.7.3</version>
</dependency>

• add a cache

@EnableCaching
public class CacheConfiguration{}
 
@Repository
public interface PersonRepository extends JpaRepository<Person, UUID> {
  @Cacheable("personCache")
  List<Person> findByName(String name);
}