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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:
...
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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).
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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$Holder classes to contain all necessary methods used by the application.
Static Dump
During the trial run (with -XX:DumpLoadedClassList=classlist), we can save the list of LambdaForm handlers using a special syntax like:
...
- http://cr.openjdk.java.net/~minqi/2020/8247536/webrev-01/
- The generation of the customized DirectMethodHandle$Holder is done in MetaspaceShared::regenerate_holder_classes().
Dynamic Dump
- (This will be done in a follow-up RFE)
- 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
...
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