Bridging the Observability Gap for Java and Scripting Applications

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DTrace: Bridging the Observability Gap for Java and Scripting ApplicationsAlex Peng Technology Evangelist Sun Microsystems1

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Agenda Introduction to DTrace The DVM provider The hotspot provider Scripting language providers DTrace and MySQL Summary and Resources

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Traditional Analysis: System Observability My application doesn't work or doesn't perform

as well as it needs to

What tools do I have? What information can I get? How can I relate this to the problem?

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What is instrumentation?? Question: How do you typically do

instrumentation?? Answer:

#ifdef DEBUG printf(“Value of Q:%d”, Q);

Question: What do you need to do to add new

instrumentation? Answer: edit, compile, link, deploy ........ the system and your own code??

What if you could do this dynamically, both on4

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DTrace Approach My application doesn't work or doesn't perform as

well as it needs to

hypothesis→ instrumentation→ data gathering→ analysis→ hypothesis ........

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DTrace Basics DTrace is a dynamic instrumentation framework

that was introduced in Solaris 10 Allows for dynamic instrumentation of the OS and applications (including Java-based applications as well as some scripting languages, as we will see later) Available out of the box; a typical system has more than 50,000 probes> Other systems also instrumented

Includes D, a dynamically interpreted language,

that allows for arbitrary actions and predicates

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DTrace Basics Designed explicitly for use on production

systems Zero performance impact when not in use Completely safe; no way to cause panics, crashes, data corruption or pathological performance degradation Powerful data management primitives eliminate need for most postprocessing

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DTrace Terminology Probe> A place of interest in the system where we can make observations Provider> Instruments a particular area of the system> Makes probes available> Transfers control into the DTrace framework when an enabled probe is hit.

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D Language - Format. A D script consist of one or more statements A statement has the following structure:

probe description/ predicate/{ action statements}

#!/usr/sbin/dtrace -s syscall:::entry/execname==”bash”/{ printf(“%s called\n”, probefunc);}

When a probe fires the actions are executed if

the predicate evaluates true Example,“Print all the system calls executed by bash”

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DTrace Terminology Aggregation> Often, patterns are more interesting than individual datum> aggregate data together to look for trends> result of an aggregation function (count(), sum(), max(), avg(), quantize()) associated with arbitrary key in an array

syscall::write:entry{@writes[execname, pid]= sum(arg2);}10

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jstack() Action

jstack action prints mixed mode stack trace Both Java frames and native (C/C++) frames are shown Only JVM versions 5.0_01 and ab

ove are supported> jstack shows hex numbers for JVM versions before

5.0_01

First

#!/usr/sbin/dtrace -s syscall::pollsys:entry/ pid==$1/ argument limits number of frames{

shown

jstack(50, string Second argument changes 8192);size} jstackstrsize pragma/-x to increase buffer for all jstack

calls

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The dvm Provider project to add DTrace support> 1.4.2 (libdvmpi.so)> 1.5 (libdvmti.so)> https:/// Download shared libs Add location of libs to LD_LIBRARY_PATH

variable Set JAVA_TOOL_OPTIONS to -Xrundvmti:all

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The dvm Provider: Probes dvm probes and their signatures:vm-init(), vm-death() thread-start(char *thread_name), thread-end() class-load(char *class_name) class-unload(char *class_name) gc-start(), gc-finish() gc-stats(long used_objects, long used_object_space) object-alloc(char *class_name, long size) object-free(char *class_name) method__entry(char *class_name, char *method_name, char *method_signature) method__return(char *class_name, char *method_name, char *method_signature)13

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The dvm Provider: Methods Count methods called

#!/usr/sbin/dtrace -s dvm$target:::method-entry{@[copyinstr(arg0),copyinstr(arg1)]= count();}

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DTrace in JDK 6 JDK 6 supports DTrace“out of the box” hotspot provider implements all dvm probes plus

extensions:

> Method compilation (method-compile-begin/end)> Compiled method load/unload(compiled-method-load/

unload)> JNI method probes.> DTrace probes as entry and return from each JNI method. Strings are now unterminated UTF-8 data.

Always use associated length value with copyinstr().

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JDK 6 DTrace Usage Certain probes are expensive Turned off by default> object-alloc, method-entry, method-return> All monitor probes: monitor-wait, etc. Use command line flag to enable if required> -XX:+ExtendedDTraceProbes (all expensive probes) Selective probe enablement (less impact)> -XX:+DTraceAllocProbes> -XX:+DTraceMethodProbes> -XX:+DTraceMonitorProbes

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JDK6 hotspot_jni Provider Probes for Java Native Interface (JNI) Located at entry/return points of all JNI

functions Probe arguments are same as corresponding JNI function arguments (for _entry probes) For XXX_return probes, probe argument is return value Examples:

hotspot_jni$1:::GetPrimitiveArrayCritical_entry hotspot_jni$1:::GetPrimitiveArrayCritical_return17

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Hotspot Probe Demo

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DTrace and Scripting Language DTrace providers are being developed for

scripting languages Examples>>>>>

Python PHP JavaScript in Firefox 3.0 Ruby More to come

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DTrace and Python Two probes, simple function entry/exit> function-entry> function-return Three arguments available> arg1: filename> arg2: subroutine name> arg3: line number

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DTrace and Python ustack() works well on these probes These probes are available in OpenSolaris

2008.05 and later DTrace toolkit

has a set of scripts for Python

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