Consuming stack traces noticeably slower in Java 1

问题:

I was comparing the performance of JDK 8 and 11 using jmh 1.21 when I ran across some surprising numbers:

Java version: 1.8.0_192, vendor: Oracle Corporation

Benchmark                              Mode  Cnt      Score    Error  Units
MyBenchmark.throwAndConsumeStacktrace  avgt   25  21525.584 ± 58.957  ns/op


Java version: 9.0.4, vendor: Oracle Corporation

Benchmark                              Mode  Cnt      Score     Error  Units
MyBenchmark.throwAndConsumeStacktrace  avgt   25  28243.899 ± 498.173  ns/op


Java version: 10.0.2, vendor: Oracle Corporation

Benchmark                              Mode  Cnt      Score     Error  Units
MyBenchmark.throwAndConsumeStacktrace  avgt   25  28499.736 ± 215.837  ns/op


Java version: 11.0.1, vendor: Oracle Corporation

Benchmark                              Mode  Cnt      Score      Error  Units
MyBenchmark.throwAndConsumeStacktrace  avgt   25  48535.766 ± 2175.753  ns/op

OpenJDK 11 and 12 perform similar to OracleJDK 11. I have omitted their numbers for the sake of brevity.

I understand that microbenchmarks do not indicate the performance behavior of real-life applications. Still, I'm curious where this difference is coming from. Any ideas?

---

Here is the benchmark in its entirety:

pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>jmh</groupId>
    <artifactId>consume-stacktrace</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>
    <name>JMH benchmark sample: Java</name>

    <dependencies>
        <dependency>
            <groupId>org.openjdk.jmh</groupId>
            <artifactId>jmh-core</artifactId>
            <version>${jmh.version}</version>
        </dependency>
        <dependency>
            <groupId>org.openjdk.jmh</groupId>
            <artifactId>jmh-generator-annprocess</artifactId>
            <version>${jmh.version}</version>
            <scope>provided</scope>
        </dependency>
    </dependencies>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <jmh.version>1.21</jmh.version>
        <javac.target>1.8</javac.target>
        <uberjar.name>benchmarks</uberjar.name>
    </properties>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-enforcer-plugin</artifactId>
                <version>1.4.1</version>
                <executions>
                    <execution>
                        <id>enforce-versions</id>
                        <goals>
                            <goal>enforce</goal>
                        </goals>
                        <configuration>
                            <rules>
                                <requireMavenVersion>
                                    <version>3.0</version>
                                </requireMavenVersion>
                            </rules>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.0</version>
                <configuration>
                    <compilerVersion>${javac.target}</compilerVersion>
                    <source>${javac.target}</source>
                    <target>${javac.target}</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>3.2.1</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <finalName>${uberjar.name}</finalName>
                            <transformers>
                                <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                                    <mainClass>org.openjdk.jmh.Main</mainClass>
                                </transformer>
                            </transformers>
                            <filters>
                                <filter>
                                    <!--
                                            Shading signed JARs will fail without this.
                                            http://stackoverflow.com/questions/999489/invalid-signature-file-when-attempting-to-run-a-jar
                                    -->
                                    <artifact>*:*</artifact>
                                    <excludes>
                                        <exclude>META-INF/*.SF</exclude>
                                        <exclude>META-INF/*.DSA</exclude>
                                        <exclude>META-INF/*.RSA</exclude>
                                    </excludes>
                                </filter>
                            </filters>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
        <pluginManagement>
            <plugins>
                <plugin>
                    <artifactId>maven-clean-plugin</artifactId>
                    <version>2.6.1</version>
                </plugin>
                <plugin>
                    <artifactId>maven-deploy-plugin</artifactId>
                    <version>2.8.2</version>
                </plugin>
                <plugin>
                    <artifactId>maven-install-plugin</artifactId>
                    <version>2.5.2</version>
                </plugin>
                <plugin>
                    <artifactId>maven-jar-plugin</artifactId>
                    <version>3.1.0</version>
                </plugin>
                <plugin>
                    <artifactId>maven-javadoc-plugin</artifactId>
                    <version>3.0.0</version>
                </plugin>
                <plugin>
                    <artifactId>maven-resources-plugin</artifactId>
                    <version>3.1.0</version>
                </plugin>
                <plugin>
                    <artifactId>maven-site-plugin</artifactId>
                    <version>3.7.1</version>
                </plugin>
                <plugin>
                    <artifactId>maven-source-plugin</artifactId>
                    <version>3.0.1</version>
                </plugin>
                <plugin>
                    <artifactId>maven-surefire-plugin</artifactId>
                    <version>2.22.0</version>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>
</project>

src/main/java/jmh/MyBenchmark.java:

package jmh;

import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.Mode;
import org.openjdk.jmh.annotations.OutputTimeUnit;
import org.openjdk.jmh.infra.Blackhole;

import java.io.PrintWriter;
import java.io.StringWriter;
import java.util.concurrent.TimeUnit;

@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public class MyBenchmark
{
    @Benchmark
    public void throwAndConsumeStacktrace(Blackhole bh)
    {
        try
        {
            throw new IllegalArgumentException("I love benchmarks");
        }
        catch (IllegalArgumentException e)
        {
            StringWriter sw = new StringWriter();
            e.printStackTrace(new PrintWriter(sw));
            bh.consume(sw.toString());
        }
    }
}

Here is the Windows-specific script I use. It should be trivial to translate it to other platforms:

set JAVA_HOME=C:\Program Files\Java\jdk1.8.0_192
call mvn -V -Djavac.target=1.8 clean install
"%JAVA_HOME%\bin\java" -jar target\benchmarks.jar

set JAVA_HOME=C:\Program Files\Java\jdk-9.0.4
call mvn -V -Djavac.target=9 clean install
"%JAVA_HOME%\bin\java" -jar target\benchmarks.jar

set JAVA_HOME=C:\Program Files\Java\jdk-10.0.2
call mvn -V -Djavac.target=10 clean install
"%JAVA_HOME%\bin\java" -jar target\benchmarks.jar

set JAVA_HOME=C:\Program Files\Java\oracle-11.0.1
call mvn -V -Djavac.target=11 clean install
"%JAVA_HOME%\bin\java" -jar target\benchmarks.jar

My runtime environment is:

Apache Maven 3.6.0 (97c98ec64a1fdfee7767ce5ffb20918da4f719f3; 2018-10-24T14:41:47-04:00)
Maven home: C:\Program Files\apache-maven-3.6.0\bin\..
Default locale: en_CA, platform encoding: Cp1252
OS name: "windows 10", version: "10.0", arch: "amd64", family: "windows"

More specifically, I am running Microsoft Windows [Version 10.0.17763.195].

回答1:

I investigated the issue with async-profiler which can draw cool flame graphs demonstrating where the CPU time is spent.

As @AlekseyShipilev pointed out, the slowdown between JDK 8 and JDK 9 is mainly the result of StackWalker changes. Also G1 has become the default GC since JDK 9. If we explicitly set -XX:+UseParallelGC (default in JDK 8), the scores will be slightly better.

But the most interesting part is the slowdown in JDK 11.
Here is what async-profiler shows (clickable SVG).

The main difference between two profiles is in the size of java_lang_Throwable::get_stack_trace_elements block, which is dominated by StringTable::intern. Apparently StringTable::intern takes much longer on JDK 11.

Let's zoom in:

Note that StringTable::intern in JDK 11 calls do_intern which in turn allocates a new java.lang.String object. Looks suspicious. Nothing of this kind is seen in JDK 10 profile. Time to look in the source code.

stringTable.cpp (JDK 11)

oop StringTable::intern(Handle string_or_null_h, jchar* name, int len, TRAPS) {
  // shared table always uses java_lang_String::hash_code
  unsigned int hash = java_lang_String::hash_code(name, len);
  oop found_string = StringTable::the_table()->lookup_shared(name, len, hash);
  if (found_string != NULL) {
    return found_string;
  }
  if (StringTable::_alt_hash) {
    hash = hash_string(name, len, true);
  }
  return StringTable::the_table()->do_intern(string_or_null_h, name, len,
                                       |     hash, CHECK_NULL);
}                                      |
                       ----------------
                      |
                      v
oop StringTable::do_intern(Handle string_or_null_h, const jchar* name,
                           int len, uintx hash, TRAPS) {
  HandleMark hm(THREAD);  // cleanup strings created
  Handle string_h;

  if (!string_or_null_h.is_null()) {
    string_h = string_or_null_h;
  } else {
    string_h = java_lang_String::create_from_unicode(name, len, CHECK_NULL);
  }

The function in JDK 11 first looks for a string in the shared StringTable, does not find it, then goes to do_intern and immediately creates a new String object.

In JDK 10 sources after a call to lookup_shared there was an additional lookup in the main table which returned the existing string without creation of a new object:

  found_string = the_table()->lookup_in_main_table(index, name, len, hashValue);

This refactoring was a result of JDK-8195097 "Make it possible to process StringTable outside safepoint".

TL;DR While interning method names in JDK 11, HotSpot creates redundant String objects. This has happened after JDK-8195097.

回答2:

I suspect this is due to several changes.

8->9 regression happened while switching to StackWalker for generating the stack traces (JDK-8150778). Unfortunately, this made VM native code intern a lot of strings, and StringTable becomes the bottleneck. If you profile OP's benchmark, you will see the profile like in JDK-8151751. It should be enough to perf record -g the entire JVM that runs the benchmark, and then look into perf report. (Hint, hint, you can do it yourself next time!)

And 10->11 regression must have happened later. I suspect this is due to StringTable preparations for switching to fully concurrent hash table (JDK-8195100, which, as Claes points out, is not entirely in 11) or something else (class data sharing changes?).

Either way, interning on fast path is a bad idea, and patch for JDK-8151751 should have dealt with both regressions.

Watch this:

8u191: 15108 ± 99 ns/op [so far so good]

-   54.55%     0.37%  java     libjvm.so           [.] JVM_GetStackTraceElement 
   - 54.18% JVM_GetStackTraceElement                          
      - 52.22% java_lang_Throwable::get_stack_trace_element   
         - 48.23% java_lang_StackTraceElement::create         
            - 17.82% StringTable::intern                      
            - 13.92% StringTable::intern                      
            - 4.83% Klass::external_name                      
            + 3.41% Method::line_number_from_bci              

"head": 22382 ± 134 ns/op [regression]

-   69.79%     0.05%  org.sample.MyBe  libjvm.so  [.] JVM_InitStackTraceElement
   - 69.73% JVM_InitStackTraceElementArray                    
      - 69.14% java_lang_Throwable::get_stack_trace_elements  
         - 66.86% java_lang_StackTraceElement::fill_in        
            - 38.48% StringTable::intern                      
            - 21.81% StringTable::intern                      
            - 2.21% Klass::external_name                      
              1.82% Method::line_number_from_bci              
              0.97% AccessInternal::PostRuntimeDispatch<G1BarrierSet::AccessBarrier<573

"head" + JDK-8151751 patch: 7511 ± 26 ns/op [woot, even better than 8u]

-   22.53%     0.12%  org.sample.MyBe  libjvm.so  [.] JVM_InitStackTraceElement
   - 22.40% JVM_InitStackTraceElementArray                    
      - 20.25% java_lang_Throwable::get_stack_trace_elements  
         - 12.69% java_lang_StackTraceElement::fill_in        
            + 6.86% Method::line_number_from_bci              
              2.08% AccessInternal::PostRuntimeDispatch<G1BarrierSet::AccessBarrier
           2.24% InstanceKlass::method_with_orig_idnum        
           1.03% Handle::Handle