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Node.js Postmortem Working Group Update
Yunong Xiao, NetflixMichael Dawson, IBM
About Michael
About The Postmortem Workgroup
Howard Hellyer@hhellyer
David Pacheco@davepacheco
Julien Gilli@mistredjules
Michael Dawson@mhdawson
Chris Bailey@seabaylea
Daniel Khan@danielkhan
Joshua Clulow@jclulow
Yunong Xiao@yunong
James Bellenger@jbellenger
Bradley Meck@bmeck
Luca Maraschi@lucamaraschi
David Clements@davidmarkclements
Richard Chamberlain@rnchamberlain
Mission Statement
The working group is dedicated to the support and improvement of postmortem debugging for Node.js.
Debugging Node.js
Debugging Node.js
Test Enviro
nment
What about production?
“The method described in this article was designed to provide a core dump… with a minimal impact on the spacecraft… as the resumption of data acquisition from the spacecraft is the highest priority.”
- Chafin, R. "Pioneer F & G Telemetry and Command Processor Core Dump Program." JPL Technical Report XVI, no. 32-1526 (1971): 174.
Core Dumps: Brief History
● Magnetic core memory● Dump out the contents
of “core” memory for debugging
● “Core dump” was coined● Initially printed on paper● Postmortem debugging
was born
Production Constraints
● Uptime is critical
● Not easily reproducible
● Can’t simulate environment
● Resume normal operations ASAP
Postmortem Debugging
└─[0] <> node --abort_on_uncaught_exception throw.jsUncaught Error
FROMObject.<anonymous> (/Users/yunong/throw.js:1:63)Module._compile (module.js:435:26)Object.Module._extensions..js (module.js:442:10)Module.load (module.js:356:32)Function.Module._load (module.js:311:12)Function.Module.runMain (module.js:467:10)startup (node.js:134:18)node.js:961:3
[1] 4131 illegal hardware instruction (core dumped) node --abort_on_uncaught_exception throw.js
Where: Inspect stack trace
Why: Inspect heap and stack variable state
Generate Core Dump Ad-hoc
root@demo:~# gcore `pgrep node`[Thread debugging using libthread_db enabled]Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".[New Thread 0x7facaeffd700 (LWP 5650)][New Thread 0x7facaf7fe700 (LWP 5649)][New Thread 0x7facaffff700 (LWP 5648)][New Thread 0x7facbc967700 (LWP 5647)][New Thread 0x7facbd168700 (LWP 5617)][New Thread 0x7facbd969700 (LWP 5616)][New Thread 0x7facbe16a700 (LWP 5615)][New Thread 0x7facbe96b700 (LWP 5614)]0x00007facbea5b5a9 in syscall () from /lib/x86_64-linux-gnu/libc.so.6Saved corefile core.5602
Example
Netflix API Request
Node to API RPC
[2016-09-09T16:25:48.388Z] WARN: reactive socket/rs-pool/17352 on lgud-yunong:
TcpLoadBalancer._connect: no more free connections
Postmortem Debugging
Connection Pool State> ::findjsobjects -p _connections | ::jsprint{ "connected": { }, "connecting": {}, "free": { "100.82.188.185:7001": [...], "100.82.37.181:7001": [...], "100.82.41.121:7001": [...], "100.82.102.157:7001": [...], "100.82.106.115:7001": [...], "100.82.129.239:7001": [...], "100.82.102.158:7001": [...], "100.82.74.237:7001": [...], ... }}
Postmortem Debugging is Critical to Large Scale Production Node Deployments
Postmortem WGgithub.com/nodejs/post-mortem/
Postmortem WG - Mission
Guide improvements in postmortem
● Interfaces/APIs ● Dump formats ● Tools and Techniques
State of key tools today
Heap dump - snapshot of heap
● heapdump module - https://github.com/bnoordhuis/node-heapdump● Chrome developer tools● Limitations
● Need to modify application● Slow to generate (minutes or hours)● O(N) memory usage ● Limited content● Output is large
Talk about IDDE, MDB, LDDB, node-heapdump
pros/cons
Low level, hard to use, not x-platform, new(lddb), slow, can’t use in prod...
State of key tools today
Core dump - memory image
● Creation○ Crash, signal○ --abort-on-uncaught-exception○ Fast (relative to heap dumps) ○ Size matches process memory
● OS debuggers○ Examination at C/C++ or assembler level○ No knowledge of Node/v8 structures
● Node core dump inspectors○ MDB (limited platform support)○ IDDE (IBM SDK specific)○ LLNODE (newer, less complete)
Example commandsMDB_V8 command LLNODE command IDDE
Print a stack trace jsstack, jsframe v8 bt !stack, !frame
Find objects findjsobjects v8 findjsobjects, v8 findjsinstances <type>
!jslistobjects!jsgroupobjects!jsfindbyproperty!jsobjectsmatching
Print an object jsprint v8 inspect !jsobject
Print function source jssource v8 source (prints source for a stack frame)
!jsobject, !string + work
Find constructor for an object
jsconstructor n/a !jsconstructor
Print elements of a FixedArray
v8array v8 inspect <instance> !array
Find native memory backing a buffer
nodebuffer v8 inspect <instance> !nodebuffers
How to make this better?
● Improve ease of use● Common APIs to introspect dumps● Cross platform support● Common command set● Lightweight dump
The Postmortem WG is working on...
Common Heap Dump Format
Improved Core Dump Analysis
● Library in C & JS● Tools: mdb_v8(mdb), llnode(lldb), ...
Node Report
Common Heap Dump Format
Enabler for new tools
Generation
● mdb● llnode
Consumption
● Conversion to existing v8 format - > chrome dev tools● C/Javascript APIs
Core Dump Analysis
Currently working on
● Platform coverage● Re-use of command implementation● Common APIs
Soon to benodejs/llnode !
https://github.com/nodejs/post-mortem/issues/37
Working to get to….
Node Report
Lightweight Dump
● Fast● Small● Human readable● Key information to start investigating● Triggers: exception, fatal error, signal, JavaScript API
NodeReportexample - heap out of memory error
NodeReport content:● Event summary● Node.js and OS versions● JavaScript stack trace● Native stack trace● Heap and GC statistics● Resource usage● libuv handle summary● Environment variables● OS ulimit settings
Javascript API
API in Javascript
● More accessible● Leverages
○ llnode ○ Common Heap Dump (future)
JavaScript API - example application
Summary
What is postmortem debugging
Example of where it’s helpful
Activities of the working group
● Common heap format● APIs (C/JS)● Tools(lldb, mdb_v8, NodeReport)
Get Involved !
Great chance to learn
● Low level machine details● Key debugging techniques● Different platforms/operating systems
Where
● Most work done through GitHub issues/Pull Requests● http://github.com/nodejs/post-mortem/
Postmortem Debugging is Critical to Large Scale Production Node Deployments
Some production problems are otherwise impossible
Save complete process state for debugging later
Copyrights and Trademarks
IBM, the IBM logo, ibm.com are trademarks or registeredtrademarks of International Business Machines Corp.,
registered in many jurisdictions worldwide. Other product andservice names might be trademarks of IBM or other companies.
A current list of IBM trademarks is available on the Web at“Copyright and trademark information” at
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Node.js is an official trademark of Joyent. IBM SDK for Node.js is not formally related to or endorsed by the official Joyent Node.js open source or commercial project.
Java, JavaScript and all Java-based trademarks and logos are trademarks or registered trademarks of Oracle and/or its affiliates.
Linux is a registered trademark of Linus Torvalds in the United States, other countries, or both.Microsoft, Windows, Windows NT, and the Windows logo are trademarks of Microsoft Corporation in the United
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