Draft JEP: Efficient Stack Walking API
jeremymanson at google.com
Mon Jul 7 16:55:49 UTC 2014
I don't know if Mandy's draft JEP has gotten any love, but this is
something that has (in the past) been a major CPU cycle consumer for us,
and we've had to invent / reinvent many wheels to fix it internally, so
we'd love to see a principled solution.
A couple of notes:
- A large percentage of the time, you just want to find one of:
1) The direct caller of the method,
2) The first caller outside a given package.
We added a CallerFinder API that basically looks like this:
// Finds the caller of the invoking method in the current stack that isn't
in one of the excluded classes
public static StackTraceElement findCaller(Class<?>... excludedClasses);
// Finds the first caller of a given class
public static StackTraceElement findCallerOf(Class<?>... classesToFind);
This isn't the ideal API (it is more the one that happened to be convenient
when we threw together the class), but it gets the vast majority of use
2) Even with a super-efficient stack walker, anyone who uses the
java.util.logging framework pervasively is going to see a lot of CPU cycles
consumed by determining the caller. We've had a lot of luck minimizing
this by using a bytecode rewriter to change callers of log(msg) to
log(sourceClass, sourceMethod, msg). This is almost certainly something
that could be done (even in a principled way!) by the VM; improvements to
CPU usage in such apps have been dramatic.
On Sun, Mar 30, 2014 at 4:02 PM, Mandy Chung <mandy.chung at oracle.com> wrote:
> Below is a draft JEP we are considering submitting for JDK 9.
> Title: Efficient API for Stack Walking
> Define a standard API for stack walking that will be efficient and
> It is not a goal for this API be easy to use via Reflection for example
> use in code that is compiled for an older JDK.
> There is no standard API to obtain information about the caller's class
> and traverse the execution stack in a performant way. Existing libraries
> and frameworks such as Log4j and Groovy have to resort to using the
> JDK internal API `sun.reflect.Reflection.getCallerClass(int depth)`.
> This JEP proposes to define a standard API for stack walking that will
> be efficient and performant and also enable the implementation up
> level the stack walk machinery from the VM to Java and replaces
> the current mechanism of `Throwable.fillInStackTrace.
> There is no standard API to traverse certain frames on the execution
> stack efficiently and access the Class instance of each frame.
> There are APIs that allow to access the stack trace information:
> - `Throwable.getStackTrace()` and `Thread.getStackTrace()` that returns
> an array of `StackTraceElement` which contains the classname
> and method name of a stack trace.
> - `SecurityManager.getClassContext()` which is a protected method
> such that only `SecurityManager` subclass can access the class
> These APIs require the VM to eagerly capture a snapshot of the entire
> stack trace and returns the information representing the entire stack.
> There is no other way to avoid the cost to examine all frames if
> the caller is only interested in the top few frames on the stack.
> Both `Throwable.getStackTrace()` and `Thread.getStackTrace()` methods
> return an array of `StackTraceElement` that contains the classname and
> method name of a stack frame but the `Class` instance.
> In fact, for applications interested in the entire stack, the specification
> allows VM implementation to omit some frames in the stack for performance.
> In other words, `Thread.getStackTrace()` may return a partial stack trace.
> These APIs do not satisfy the use cases that currently depend on
> the `getCallerClass(int depth)` method or its performance overhead
> is intolerable. The use cases include:
> - JDK caller-sensitive APIs look up its immediate caller's class
> which will be used to determine the behavior of the API. For example
> `Class.forName(String classname)` and
> `ResourceBundle.getBundle(String rbname)` methods use the immediate
> caller's class loader to load a class and a resource bundle
> `Class.getMethod` etc will use the immediate caller's class loader
> to determine the security checks to be performed.
> - `java.util.logging`, Log4j and Groovy runtime filter the intermediary
> stack frames (typically implementation-specific and reflection frames)
> and find the caller's class to be used by the runtime of such library
> or framework.
> - Traverse the entire stack trace or the stack trace of a `Throwbale`
> and obtain additional information about classes for enhanced
> diagnosibility in addition to the class and method name.
> This JEP will define a stack walk API that allows laziness, frame
> supports short reaches to stop at a frame matching some criteria
> as well as long reaches to traverse the entire stack trace. This would
> need the JVM to provide a flexible mechanism to traverse and materialize
> the specific stack frame information to be used and allow efficient
> lazy access to additional stack frames when required.
> Native JVM transitions should be minimzed.
> The API will define how it works when running with a security manager
> that allows access to a `Class` instance
> of any frame ensuring that the security is not compromised.
> An example API to walk the stack can be like:
> Thread.walkStack(Consumer<StackFrameInfo> action, int depthLimit)
> that takes a callback to be invoked for each frame traversed. A variant
> of the walkStack method will take a predicate for stack frame filtering.
> Thread.getCaller(Function<StackFrameInfo, R> function)
> Thread.findCaller(Predicate<StackFrameInfo> predicate,
> Function<StackFrameInfo, R> function)
> finds the caller frame with or without filtering.
> Unit tests and JCK tests for the new SE API will need to be developed.
> In addition, the performance of the new API for different use cases
> will be assessed.
> - Performance/scalability: performance measurement shall be performed
> using micro-benchmarks as well as real world usage of `getCallerClass`
> replaced with the new API.
> - TCK: New JCK test cases shall be developed.
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