Test a complete failure of the JVM

Say you want to test that your application behaves correctly on restart even after the application crashes. One approach is to trigger a crash in test code and check that data is in a correctable state on restart. 
Unsafe unsafe = getUnsafe(); // use reflection
unsafe.setMemory(0, 1, (byte) 0);
This triggers a SIGSEGV 
#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00000032b967ae09, pid=17870, tid=1082034496
#
# JRE version: 7.0_01-b08
# Java VM: Java HotSpot(TM) 64-Bit Server VM (21.1-b02 mixed mode linux-amd64 compressed oops)
# Problematic frame:
# C  [libc.so.6+0x7ae09]  memset+0x9
#
If your tests still pass, you can be reasonably confident it is recoverable even on a complete failure of the JVM.

For the code for getUnsafe().

Warning: This class is appropriately named and misuse can result in a crash of the JVM. It can also change in future versions (It is more likely new methods will be added rather than removed) 

public static Unsafe getUnsafe() {
    try {
        Field field = Unsafe.class.getDeclaredField("theUnsafe");
        field.setAccessible(true);
        return (Unsafe) field.get(null);
    } catch (Exception e) {
        throw new AssertionError(e);
    }
}

Comments

  1. Unknown5 December 2011 at 17:58

    I don't think everyone knows about/has access to a getUnsafe() method, so for reference here is such a method that should work

    Unsafe getUnsafe() {
    Unsafe unsafe = null;
    try {
    Field field = sun.misc.Unsafe.class.getDeclaredField("theUnsafe");
    field.setAccessible(true);
    unsafe = (sun.misc.Unsafe) field.get(null);
    } catch (Exception e) {
    throw new AssertionError(e);
    }
    return unsafe;
    }

    ReplyDelete
  2. Przemysław Bielicki5 December 2011 at 18:30

    hi,

    what's the usefulness of such test? if the JVM crashes your app needs to reloaded, re-initialized, re-warmed-up, etc.

    "test that your application behaves correctly" - what application? when the JVM is down there is no application - it's like a machine reboot for Apache server, right?

    cheers

    ReplyDelete
  3. Peter Lawrey5 December 2011 at 19:00

    @Premyslav, The application stores its data in memory mapped files. On restart, all the data is already in memory. ;) In this case, it is to ensure that data written to memory is not lost even if the program immediately crashes.

    ReplyDelete
  4. Peter Lawrey5 December 2011 at 19:02

    @jackson, I tend not to give the source on the assumption that anyone who can't figure it out probably shouldn't be using it. ;)

    ReplyDelete
  5. Przemysław Bielicki5 December 2011 at 20:12

    OK I get it but "behaves correctly" is not enough. you should make a full scan to see if memory state is not corrupted.

    anyway, thanks for sharing this. sometimes it's quite useful to simulate unplugged power or network cable ;)

    ReplyDelete
  6. Noms6 December 2011 at 17:54

    Thanks Jackson for the code listing.

    ReplyDelete

Post a Comment

Popular posts from this blog

Java is Very Fast, If You Don’t Create Many Objects

Image
  You still have to watch how many objects you create. This article looks at a benchmark passing events over TCP/IP at 4 billion events per minute using the net.openhft.chronicle.wire.channel package in Chronicle Wire and why we still avoid object allocations..  One of the key optimisations is creating almost no garbage. Allocation is a very cheap operation and collection of very short-lived objects is also very cheap. Does this really make a difference? What difference does one small object per event (44 bytes) make to the performance in a throughput test where GC pauses are amortised? While allocation is as efficient as possible, it doesn’t avoid the memory pressure on the L1/L2 caches of your CPUs and when many cores are busy, they are contending for memory in the shared L3 cache.  Results Benchmark on a Ryzen 5950X with Ubuntu 22.10. JVM Vendor, Version No objects Throughput, Average Latency* One object per event Throughput, Average Latency* Azul Zulu 1.8.0_322 60.6 ...
Read more

System wide unique nanosecond timestamps

A Unique Identifier can be very useful for tracing. Those ids are even more useful when they contain a high-resolution timestamp.  Not only do they record the time of an event, but if unique can help trace events as they pass through the system. Such unique timestamps however can be expensive depending on how they are implemented.   This post explores a lightweight means of producing a unique, monotonically increasing system-wide nano-second resolution timestamp available in our open-source library. Uses for Unique Identifiers Unique identifiers can be useful to associate with a piece of information so that information can be referred to later unambiguously. This could be an event, a request, an order id, or a customer id. They can naturally be used as a primary key in a database or key/value store to retrieve that information later. One of the challenges of generating these identifiers is avoiding creating duplicates while not having an increasing cost.  You could ...
Read more

What does Chronicle Software do?

Image
Overview Chronicle Software is about simplifying fast data.  It is a suite of libraries to make it easier to write, monitor and tune data processing systems where performance and scalability are concerned. But its free, how do you make money, through support? We offer premium support . However, we often hear from users for the first time about a year after they have it in production.  Most users find they can support the software themselves.  It is only after a year or so, they have questions or concerns about where to take the product next.  In many cases, it is to a problem we have already solved and they just need to upgrade their software or use a new module we have added. As we don't know who is using most of our software, we can't advise them on how best to use our software and what updates or enhancements they would benefit from. We need users to contact us and ask questions.  We have a free forum , and we respond to 50% of questions in 2 hours...
Read more