Hi Peter,
The hazard to avoid is the cross package coupling that makes both
ClassLoader and
ObjectInputStream more complex; both are more than sufficiently complex
already.
Optimizing the implementations make sense if it benefits enough uses
cases and
does not make the code harder to maintain and evolve.
Have you explored any alternatives that can be confined to OIS?
For example, taking advantage the many calls to readObject within
a single OIS that are nested. Possibly the first (depth = 0) find user
class loader
can be re-used or at least to short circuit the search on subsequent
nested calls?
Or caching the classloader at each depth and only need to probe for it
if it has
not already been recorded. The StackWalker API may be useful to limit
the depth
of searches.
Thanks, Roger
On 6/11/20 2:52 PM, Peter Kessler OS wrote:
Hi Roger,
Thanks for your comments.
I am not knowingly changing the required behavior. All I am proposing is
guarding the thread stack walk if it will be a waste of effort. If an
application or framework has its own ClassLoaders, then the guard will fail
and the thread stack will be walked.
The stack walk is important, if there are user-defined ClassLoaders. A
method that uses ObjectInputStream to read objects can not know what
ClassLoader to use to resolve class references in the stream, without the
stack walk. But if there never have been any user-defined ClassLoaders
constructed, then the stack walk can never return a user-defined
ClassLoader.
If the stream reader is part of some simple application, then it may spend
a significant amount of its time uselessly walking thread stacks. (If the
stream reader is part of some simple application its author may not know to
override ObjectInputStream.resolveClass, or how to determine what
ClassLoader should load a class from the stream.) If someone were using
such an application to choose platforms, the Java platform would be at a
disadvantage.
Consider a somewhat more complex case, the SPECjbb2015 benchmark, which
claims to model a well-constructed business application on the Java
platform. SPECjbb2015 runs multiple JVMs, exchanging information via
ObjectInputStreams. Profiling shows that it spends more than 5% of its time
walking thread stacks. Yet, in spite of being a complex application,
written by experts in the Java platform, there are no user-defined
ClassLoaders. So all the time spent walking stacks is wasted time, and is
one of the reasons the servers fail their service-level agreements. We can
discuss whether SPECjbb2015 is representative of Java applications. But it
is easily available and relatively simple to run. People use benchmarks
like SPECjbb2015 to decide whether to use the Java platform for particular
tasks, and it would be unfortunate if we did not make the Java platform as
good as it could be.
... peter
-----Original Message-----
From: core-libs-dev <[email protected]> on behalf of Roger Riggs
<[email protected]>
Organization: Java Products Group, Oracle
Date: Tuesday, June 9, 2020 at 6:43 AM
To: "[email protected]" <[email protected]>
Subject: Re: [PATCH] 8246633: Improve the performance of
ObjectInputStream.resolveClass(ObjectStreamClass)
Hi Peter,
I'd like to understand the scope and impact of the problem before
jumping to a solution.
For specific applications, overriding ObjectInputStream.resolveClass to
handle the class
lookup can handle the case as expeditiously as is necessary.
What application uses cases are impacted and how prevelant are they?
Many, many frameworks and applications create classloaders for their own
purposes and
take advantage of the required behavior.
Thanks, Roger
On 6/4/20 7:08 PM, Peter Kessler OS wrote:
> ObjectInputStream.resolveClass(ObjectStreamClass) is specified to find "the
first class loader on the current thread's stack (starting from the currently executing
method) that is neither the platform class loader nor its ancestor." Finding that
class loader is done with a virtual frame walk in JVM_LatestUserDefinedLoader(JNIEnv *env).
The virtual frame walk is expensive. If an application does not define any ClassLoaders,
then the virtual frame walk will always find the system class loader.
>
> If an application does a significant amount of reading from an
ObjectInputStream, the cost of
ObjectInputStream.resolveClass(ObjectInputStreamClass) can be considerable. In a
sample application, that method (and the methods it calls) have been measured to
consume 5+% of the CPU cycles.
>
> The proposal is to note the construction of any user-defined
ClassLoader, and if none has been constructed, avoid the virtual frame walk
returning ClassLoader.getSystemClassLoader(). But if any user-defined ClassLoader
has been constructed, use the virtual frame walk to find the first user-defined
ClassLoader on the thread stack.
>
> User-defined ClassLoaders could be distinguished from non-user-defined
ClassLoaders in several ways. The ClassLoader class hierarchy could be changed to
make non-user-defined ClassLoaders inherit from a marker class. There is already
a jdk.internal.loader.BuiltinClassLoader but it is used for module (and resource)
loading. BuiltinClassLoader is not a super-class of
jdk.internal.reflect.DelegatingClassLoader, which is used to load reflection
classes, and reflection methods are also ignored by the virtual frame walk.
ClassLoaders could have a boolean that distinguished whether they are user-defined
or not. That would require a change to ClassLoader API. There are other ways to
distinguish user-defined ClassLoaders. The proposal is to keep the decoration of
the ClassLoaders out of the instances, so that it is only visible to the code that
needs it.
>
> The proposal is that each non-user-defined ClassLoader have a static
block that registers the class as a non-user-defined ClassLoader. Then in the
base ClassLoader constructor check if the ClassLoader being constructed is a
user-defined ClassLoader and if so set a boolean that acts as the guard on the
virtual frame walk. If additional non-user-defined ClassLoaders are declared in
the future beyond the ones identified in this patch, they can be added to the
registry.
>
> There are currently 4 non-user-defined ClassLoader classes, so the
registry is not difficult to maintain. Nor is the registry difficult to search
when ClassLoaders are constructed. The guard boolean that records whether a
user-defined ClassLoader has been constructed transitions from false to true and
never becomes false again. (An extension to transition the boolean back to false
when a user-defined ClassLoader becomes unreachable is beyond the scope of this
proposal.)
>
> This proposal slows loading of non-user-defined ClassLoaders by the time
it takes to register them. It slows ClassLoader construction by the time it takes
to consult the registry and conditionally set the guard boolean. At each
invocation of ObjectInputStream.resolveClass(ObjectInputStreamClass), the guard
boolean is used to possibly avoid the virtual frame walk.
>
> Tested with `make run-test-tier1` on Linux (CentOS 7) on both aarch64
and x86_64. I had one failure on each of the runs
>
> ACTION: main -- Failed. Execution failed: `main' threw
exception: java.lang.Error: TESTBUG: the library has not been found in
${Repo}/build/linux-aarch64-server-release/images/test/hotspot/jtreg/native
> REASON: User specified action: run main/native GTestWrapper
>
> on both plaforms. But I get the same failure when I run the tier1 tests
on clones of OpenJDK-15+25 on both aarch64 and x86_64.
>
> Running a sample (representative?) application with the Oracle Developer
Studio analyzer shows a performance comparison of
>
> Stack Fragment sorted by metric: Attributed Total CPU Time
> baseline.er experiment.er
> Attributed Attributed Name
> Total CPU Time Total CPU Time
> sec. sec.
> ================================== Callers
> 3459.210 29.931
java.io.ObjectInputStream.readOrdinaryObject(boolean)
> 1139.727 3.532
java.io.ObjectInputStream.readArray(boolean)
> 875.262 9.116
java.io.ObjectInputStream.readNonProxyDesc(boolean)
>
> ================================== Stack Fragment
>
java.io.ObjectInputStream.readClassDesc(boolean)
>
java.io.ObjectInputStream.readNonProxyDesc(boolean)
>
java.io.ObjectInputStream.resolveClass(java.io.ObjectStreamClass)
>
java.io.ObjectInputStream.latestUserDefinedLoader()
> 4.173 3.953
jdk.internal.misc.VM.latestUserDefinedLoader()
>
> ================================== Callees
> 5470.026 0.
jdk.internal.misc.VM.latestUserDefinedLoader0()
> 0. 38.627
java.lang.ClassLoader.getSystemClassLoader()
>
> The `hg export -g` below is with respect to OpenJDK-15+25.
>
> Thank you for your review comments. I will need a sponsor to get this
change into the repository.
>
> ... peter
>
> # HG changeset patch
> # User Peter Kessler <[email protected]>
> # Date 1591310467 25200
> # Thu Jun 04 15:41:07 2020 -0700
> # Node ID 9a39488182c1dfc5bc7bb41d562d7ef16ee657f6
> # Parent 90b266a84c06f1b3dc0ed8767856793e8c1c357e
> Improve the performance of ObjectInputStream.resolveClass
>
> diff --git a/src/java.base/share/classes/java/lang/ClassLoader.java
b/src/java.base/share/classes/java/lang/ClassLoader.java
> --- a/src/java.base/share/classes/java/lang/ClassLoader.java
> +++ b/src/java.base/share/classes/java/lang/ClassLoader.java
> @@ -385,6 +385,8 @@
> }
> this.package2certs = new ConcurrentHashMap<>();
> this.nameAndId = nameAndId(this);
> + /* Note the construction of a ClassLoader. */
> + VM.noteClassLoaderConstruction(this.getClass());
> }
>
> /**
> diff --git
a/src/java.base/share/classes/jdk/internal/loader/ClassLoaders.java
b/src/java.base/share/classes/jdk/internal/loader/ClassLoaders.java
> --- a/src/java.base/share/classes/jdk/internal/loader/ClassLoaders.java
> +++ b/src/java.base/share/classes/jdk/internal/loader/ClassLoaders.java
> @@ -117,6 +117,11 @@
> protected Class<?> loadClassOrNull(String cn, boolean resolve)
{
> return JLA.findBootstrapClassOrNull(this, cn);
> }
> +
> + static {
> + /* Register this ClassLoader as not a user-defined
ClassLoader. */
> + VM.registerNotUserDefinedClassLoader(BootClassLoader.class);
> + }
> };
>
> /**
> @@ -127,6 +132,8 @@
> static {
> if (!ClassLoader.registerAsParallelCapable())
> throw new InternalError();
> + /* Register this ClassLoader as not a user-defined
ClassLoader. */
> +
VM.registerNotUserDefinedClassLoader(PlatformClassLoader.class);
> }
>
> PlatformClassLoader(BootClassLoader parent) {
> @@ -142,6 +149,8 @@
> static {
> if (!ClassLoader.registerAsParallelCapable())
> throw new InternalError();
> + /* Register this ClassLoader as not a user-defined
ClassLoader. */
> + VM.registerNotUserDefinedClassLoader(AppClassLoader.class);
> }
>
> final URLClassPath ucp;
> diff --git a/src/java.base/share/classes/jdk/internal/misc/VM.java
b/src/java.base/share/classes/jdk/internal/misc/VM.java
> --- a/src/java.base/share/classes/jdk/internal/misc/VM.java
> +++ b/src/java.base/share/classes/jdk/internal/misc/VM.java
> @@ -304,12 +304,44 @@
> private static final int JVMTI_THREAD_STATE_WAITING_INDEFINITELY =
0x0010;
> private static final int JVMTI_THREAD_STATE_WAITING_WITH_TIMEOUT =
0x0020;
>
> + /** A registry of not user-defined ClassLoaders. */
> + private static final List<Class<? extends ClassLoader>>
notUserDefinedClassLoaderRegistry =
> +
Collections.synchronizedList(new ArrayList<>());
> +
> + /** Register a ClassLoader class as being not a user-defined
ClassLoader. */
> + public static void registerNotUserDefinedClassLoader(Class<? extends
ClassLoader> classLoaderClass) {
> + notUserDefinedClassLoaderRegistry.add(classLoaderClass);
> + }
> +
> + /** A flag for whether a user-defined ClassLoaders has been
constructed. */
> + private static volatile boolean
constructedUserDefinedClassLoaderFlag = false;
> +
> + /**
> + * Note a ClassLoader construction, and record if a
> + * user-defined ClassLoader has been constructed.
> + */
> + public static void noteClassLoaderConstruction(Class<? extends
ClassLoader> classLoaderClass) {
> + /* Check if the ClassLoader class not a user-defined
ClassLoader. */
> + if
(notUserDefinedClassLoaderRegistry.contains(classLoaderClass)) {
> + return;
> + }
> + constructedUserDefinedClassLoaderFlag = true;
> + return;
> + }
> +
> /*
> * Returns the first user-defined class loader up the execution
stack,
> * or the platform class loader if only code from the platform or
> * bootstrap class loader is on the stack.
> */
> public static ClassLoader latestUserDefinedLoader() {
> + if (!constructedUserDefinedClassLoaderFlag) {
> + /*
> + * If no user-defined ClassLoader has been constructed,
> + * then I will not find a user-defined ClassLoader in the
stack.
> + */
> + return ClassLoader.getSystemClassLoader();
> + }
> ClassLoader loader = latestUserDefinedLoader0();
> return loader != null ? loader :
ClassLoader.getPlatformClassLoader();
> }
> diff --git
a/src/java.base/share/classes/jdk/internal/reflect/ClassDefiner.java
b/src/java.base/share/classes/jdk/internal/reflect/ClassDefiner.java
> --- a/src/java.base/share/classes/jdk/internal/reflect/ClassDefiner.java
> +++ b/src/java.base/share/classes/jdk/internal/reflect/ClassDefiner.java
> @@ -30,6 +30,7 @@
>
> import jdk.internal.access.JavaLangAccess;
> import jdk.internal.access.SharedSecrets;
> +import jdk.internal.misc.VM;
>
> /** Utility class which assists in calling defineClass() by
> creating a new class loader which delegates to the one needed in
> @@ -73,4 +74,9 @@
> DelegatingClassLoader(ClassLoader parent) {
> super(parent);
> }
> +
> + static {
> + /* Register this ClassLoader as not a user-defined ClassLoader.
*/
> +
VM.registerNotUserDefinedClassLoader(DelegatingClassLoader.class);
> + }
> }
>
