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Table of Contents

Goal

Improve start-up time by storing pre-generated LambdaForm handlers in an application-specific CDS archive.

Background

In JDK-8086045 (Improve the java.lang.invoke first initialization costs), the JDK can reduce the number of dynamically generated LambdaForm classes. Here's an example:

No Format
# Do this in your JDK build directory.
# The exploded JDK build:
$ jdk/bin/javac -J-verbose ~/tmp/HelloWorld.java | grep LambdaForm | grep __JVM_LookupDefineClass__ 
[0.142s][info][class,load] java.lang.invoke.LambdaForm$MH/0x0000000800066c40 source: __JVM_LookupDefineClass__
[0.143s][info][class,load] java.lang.invoke.LambdaForm$DMH/0x0000000800066040 source: __JVM_LookupDefineClass__
[0.143s][info][class,load] java.lang.invoke.LambdaForm$MH/0x0000000800066440 source: __JVM_LookupDefineClass__
[snip]

$ jdk/bin/javac -J-verbose ~/tmp/HelloWorld.java | grep LambdaForm | grep __JVM_LookupDefineClass__ | wc
    102     408   11400


# The final JDK image: 
$ images/jdk/bin/javac -J-verbose ~/tmp/HelloWorld.java | grep LambdaForm | grep __JVM_LookupDefineClass__ |  wc
     34     136    3800

...

Code Block
    private static MemberName lookupPregenerated(LambdaForm form, MethodType invokerType) {
        if (form.customized != null) {
            // No pre-generated version for customized LF
            return null;
        }
        String name = form.kind.methodName; 
        switch (form.kind) {
            [......]
            case DIRECT_INVOKE_INTERFACE:   // fall-through
            case DIRECT_INVOKE_SPECIAL:     // fall-through
            case DIRECT_INVOKE_SPECIAL_IFC: // fall-through
            case DIRECT_INVOKE_STATIC:      // fall-through
            case DIRECT_INVOKE_STATIC_INIT: // fall-through
            case DIRECT_INVOKE_VIRTUAL:     return resolveFrom(name, invokerType, DirectMethodHandle.Holder.class);
        }
        return null;


Estimated Benefits

The expected benefits will be application specific. Here's an experiment with Javac (I modified the JDK makefile to invoke com.sun.tools.javac.Main while generating default_jli_trace.txt).

Code Block
Results of " perf stat -r 50 bin/javac -J-Xshare:on -J-XX:SharedArchiveFile=javac2.jsa Bench_HelloWorld.java "
   1:   2239149872  2203293903 (-35855969)      ----     375.580   367.190 ( -8.390)      ----  
   2:   2245304165  2198900711 (-46403454)      -----    375.350   366.910 ( -8.440)      ----  
   3:   2242558895  2205167765 (-37391130)      ----     376.720   366.480 (-10.240)      ----- 
   4:   2246851428  2198990987 (-47860441)      -----    375.119   367.010 ( -8.109)      ----  
   5:   2238549008  2202480450 (-36068558)      ----     374.100   367.600 ( -6.500)      ---   
   6:   2240816859  2200725384 (-40091475)      ----     375.743   366.880 ( -8.863)      ----  
   7:   2243272464  2199639926 (-43632538)      -----    374.296   365.990 ( -8.306)      ----  
   8:   2239650882  2203211291 (-36439591)      ----     375.315   365.520 ( -9.795)      ----- 
   9:   2242434935  2202380291 (-40054644)      ----     376.052   367.305 ( -8.747)      ----  
  10:   2238725993  2201342398 (-37383595)      ----     375.478   366.743 ( -8.735)      ----  
============================================================
        2241729810  2201612439 (-40117371)      ----     375.375   366.762 ( -8.612)      ----  
instr delta =    -40117371    -1.7896%
time  delta =       -8.612 ms -2.2943%

Why do it in CDS

  • We cannot store all possible LambdaForm handlers in the standard JDK image – we cannot enumerate all possible forms that may be used by all possible apps.
  • We could also do this as part of jlink, when creating a custom JDK image. However, some users may not want to create a new JDK image.
    • Also, there's currently no work-flow to include profiling data when building a custom JDK image (although this can be changed, probably as part of Project Leyden)
  • CDS dynamic dump can be used without generating profiling data in a separate step, so the usability is better.

CDS Design

We regenerate the XXX$Holder classes to contain all necessary methods used by the application.

Static Dump

This has been implemented in JDK-8247536 (Support for pre-generated java.lang.invoke classes in CDS static archive - integrated into JDK 16).

During the trial run (with -XX:DumpLoadedClassList=classlist), we can save the list of LambdaForm handlers using a special syntax like:

...

Dynamic Dump

  • (This will be done in a follow-up RFESee JDK-8255493 (Support for pre-generated java.lang.invoke classes in CDS dynamic archive)
  • Collect a @lambda-form-invoker list (in memory) when -XX:ArchiveClassesAtExit is specified.
  • When the dynamic archive is dumped, use this list to create customized Holder classes.
  • TBD

...