Draft JEP: Efficient Stack Walking API

Mandy Chung mandy.chung at oracle.com
Tue Jul 8 03:06:55 UTC 2014

Hi Jeremy,

Thanks for the feedback and the CallerFinder API you have.

On 7/7/2014 9:55 AM, Jeremy Manson wrote:
> Hey folks,
> I don't know if Mandy's draft JEP has gotten any love,

The JEP process is in transition to 2.0 version.  Hope this JEP will 
come out soon.

> 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.

The current thinking is to allow you to find the direct caller as well 
as express the predicate for filtering that will cover these cases.

> 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 cases.

Does it use Thread.getStackTrace() to implement this CallerFinder API?   
Thread.getStackTrace or Throwable.getStackTrace both eagerly capture the 
entire stack trace that is expensive.  We want to have the VM to be able 
to only capture the stack frames that the client needs and the 
implementation as efficient as possible.

> 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.

The current LogRecord implementation calls new Throwable that has to pay 
the cost of capturing the entire stack.

>  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.

Thanks.  I'll make sure to measure and compare the performance with 
java.util.logging using the new stack walk API and also may ask your 
help to determine if you observe the performance difference comparing 
the rewritten bytecode vs the java.util.logging using the new API.


> Jeremy
> On Sun, Mar 30, 2014 at 4:02 PM, Mandy Chung <mandy.chung at oracle.com 
> <mailto:mandy.chung at oracle.com>> wrote:
>     Below is a draft JEP we are considering submitting for JDK 9.
>     Mandy
>     ----------------------------
>     Title: Efficient API for Stack Walking
>     Goal
>     ----
>     Define a standard API for stack walking that will be efficient and
>     performant.
>     Non-goal
>     --------
>     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.
>     Motivation
>     ----------
>     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.
>     Description
>     -----------
>     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
>          context.
>     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
>     respectively.
>         `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
>     filtering,
>     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.
>     Testing
>     -------
>     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.
>     Impact
>     ------
>       - 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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