Node Interactive Debugging Node.js In Production

Debugging Node.js in Production
Yunong Xiao
@yunongx
Software Engineer
Node Platform

Node.js @ Netflix
❖ 65+ Million Subscribers
❖ Website (netﬂix.com)
❖ Dynamic asset packager
❖ PaaS on Node
❖ Internal Services

–Gene Kranz, Flight Director, Apollo 13
“Let's work the problem, people. Let's not make
things any worse by guessing”

Apply the Scientific Method
1. Construct a Hypothesis
2. Collect data
3. Analyze data and draw a conclusion
4. Repeat

Production Crisis
❖ Runtime Performance
❖ Runtime Crashes
❖ Memory Leaks

Gather Request Data
http://restify.com
http://github.com/restify/node-restify
Observable REST Framework

to the Rescue
[2014-12-09T14:07:26.293Z] INFO: shakti/restify-audit/20067: handled:
200, latency=1402
(req_id=b3fa3820-7fac-11e4-8908-a5c7b70d676f, latency=1435)
GET / HTTP/1.1
host: www.netflix.com
--
HTTP/1.1 200 OK
x-netflix.client.instance: i-057e47ef
x-frame-options: DENY
content-type: text/html
--
req.timers: {
"parseBody": 700123,
"apiRpc": 701911,
"render": 400031
}

req.timers: {
“apiRPC”: 301911,
"render": 400031,
}
On CPU

CPU is Critical
❖ Node is essentially “single threaded”
❖ Cascading effect on ALL requests in process

req.timers: {
“apiRPC”: 301911,
"render": 400031,
}
Can’t process ANY other request for 1.1 seconds
On CPU

How Much Code?
$ find . -name "*.js*" | xargs cat | wc -l
6 042 301

Statistically Sample Stack Traces

Snapshot What’s Currently Executing
Stacktrace: A stack trace is a report of the active stack frames
at a certain point in time during the execution of a program.
> console.log(ex, ex.stack.split("n"))
ReferenceError: ex is not defined
at repl:1:13
at REPLServer.defaultEval (repl.js:132:27)
at bound (domain.js:254:14)
at REPLServer.runBound [as eval] (domain.js:267:12)
at REPLServer.<anonymous> (repl.js:279:12)
at REPLServer.emit (events.js:107:17)
at REPLServer.Interface._onLine (readline.js:214:10)
at REPLServer.Interface._line (readline.js:553:8)
at REPLServer.Interface._ttyWrite (readline.js:830:14)
at ReadStream.onkeypress (readline.js:109:10)

Two Problems
1) How to sample stack traces from a running
process?
2) How to do 1) without affecting the process?

Linux Perf Events
PERF(1) perf Manual PERF(1)
NAME
perf - Performance analysis tools for Linux
SYNOPSIS
perf [--version] [--help] COMMAND [ARGS]
DESCRIPTION
Performance counters for Linux are a new kernel-based subsystem
that provide a framework for all things performance analysis.
It covers hardware level (CPU/PMU, Performance Monitoring Unit)
features and software features (software counters, tracepoints)
as well.

Sample Stack Traces w/ perf(1)
# perf record -F 99 -p `pgrep -n node` -g -- sleep 30
[ perf record: Woken up 2 times to write data ]
[ perf record: Captured and wrote 0.524 MB perf.data
(~22912 samples) ]

Sample Stack Trace
ab2fee v8::internal::Heap::DeoptMarkedAllocationSites() (/apps/node/bin/
a69754 v8::internal::StackGuard::HandleInterrupts() (/apps/node/bin/node)
c9f13b v8::internal::Runtime_StackGuard(int, v8::internal::Object**
3c793e3060bb (/tmp/perf-5382.map)
3c793e3060bb (/tmp/perf-5382.map) (repeated 30 more lines)
8e6b2f v8::Function::Call(v8::Local<v8::Context>, v8::Local<v8::Value>,
int, v8::Local<v8::Value>*) (/apps/node/bin/node)
8f2281 v8::Function::Call(v8::Local<v8::Value>, int,
v8::Local<v8::Value>*) (/apps/node/bin/node)
df599a node::MakeCallback(node::Environment*, v8::Local<v8::Value>,...
df5ccb node::CheckImmediate(uv_check_s*) (/apps/node/bin/node)
fb1597 uv__run_check (/apps/node/bin/node)
fabcee uv_run (/apps/node/bin/node)
dfaa50 node::Start(int, char**) (/apps/node/bin/node)
7fcc3ef6876d __libc_start_main (/lib/x86_64-linux-gnu/libc-2.15.so)
Missing JS Frames

Why? v8 places symbols JIT(Just in
Time)

node --perf_basic_prof_only_functions
“outputs the ﬁles in a format that the existing perf tool
can consume.”

node --perf_basic_prof_only_functions
Available right now in Node v5.x
Coming soon to Node v4.3:
https://github.com/nodejs/node/pull/3609

Results
node 5382 cpu-clock:
3c793e38b0c1 LazyCompile:DELETE native runtime.js:349 (/tmp/perf-5382.map)
3c793e31981d Builtin:JSConstructStubGeneric (/tmp/perf-5382.map)
3c793ff2ca94 (/tmp/perf-5382.map)
3c793e98a10f LazyCompile:~AtlasClient._run /apps/node/webapp/node_modules/nf-atlas-client/lib/client/AtlasClient.js:85 (/tmp/
perf-5382.map)
3c793f47de29 LazyCompile:*AtlasClient.timer /apps/node/webapp/node_modules/nf-atlas-client/lib/client/AtlasClient.js:70 (/tmp/
perf-5382.map)
3c793e9eee38 LazyCompile:~fetchSingleGetCallback /apps/node/webapp/singletons/ShaktiFetcher.js:120 (/tmp/perf-5382.map)
3c793f6cffee LazyCompile:*Model.get /apps/node/webapp/node_modules/nf-models/lib/Model.js:90 (/tmp/perf-5382.map)
3c793ed3e2ad (/tmp/perf-5382.map)
3c7940e4357b Handler:ca (/tmp/perf-5382.map)
3c793f060e3c Function:~ /apps/node/webapp/node_modules/vasync/lib/vasync.js:134 (/tmp/perf-5382.map)
3c79404edbfa (/tmp/perf-5382.map)
3c79401fd3f7 (/tmp/perf-5382.map)
3c79400e307b LazyCompile:*fetchMulti /apps/node/webapp/singletons/ShaktiFetcher.js:50 (/tmp/perf-5382.map)
3c793fb9a59f LazyCompile:*fetch /apps/node/webapp/singletons/ShaktiFetcher.js:32 (/tmp/perf-5382.map)
3c793e896697 (/tmp/perf-5382.map)
3c7943aaabbe (/tmp/perf-5382.map)
3c793ef4c53c Function:~ /apps/node/webapp/node_modules/vasync/lib/vasync.js:245 (/tmp/perf-5382.map)
3c793eaf4f01 LazyCompile:* /apps/node/webapp/node_modules/nf-packager/lib/index.js:194 (/tmp/perf-5382.map)
3c793eab130a LazyCompile:processImmediate timers.js:352 (/tmp/perf-5382.map)
3c793e319f7d Builtin:JSEntryTrampoline (/tmp/perf-5382.map)
3c793e3189e2 Stub:JSEntryStub (/tmp/perf-5382.map)
a65baf v8::internal::Execution::Call(v8::internal::Isolate*, v8::internal::Handle<v8::internal::Object>,
v8::internal::Handle<v8::internal::Object>, int, v8::internal::Handle<v8::internal::Object>*, bool) (/apps/node/bin/node)
8e6b2f v8::Function::Call(v8::Local<v8::Context>, v8::Local<v8::Value>, int, v8::Local<v8::Value>*) (/apps/node/bin/node)
8f2281 v8::Function::Call(v8::Local<v8::Value>, int, v8::Local<v8::Value>*) (/apps/node/bin/node)
df599a node::MakeCallback(node::Environment*, v8::Local<v8::Value>, v8::Local<v8::Function>, int, v8::Local<v8::Value>*)
(/apps/node/bin/node)
df5ccb node::CheckImmediate(uv_check_s*) (/apps/node/bin/node)
fb1597 uv__run_check (/apps/node/bin/node)
fabcee uv_run (/apps/node/bin/node)
dfaa50 node::Start(int, char**) (/apps/node/bin/node)
7fcc3ef6876d __libc_start_main (/lib/x86_64-linux-gnu/libc-2.15.so)
)
JS Frames
Native Frames

Problem: Too Many Traces
$ cat out.nodestacks01 | grep cpu-clock | wc -l
744
$ wc -l out.nodestacks01
58116

Flamegraph
❖ Each box presents a function in
the stack (stack frame)
❖ x-axis: percent of time on CPU
❖ y-axis: stack depth
❖ colors: random, or can be a
dimension
❖ https://github.com/
brendangregg/FlameGraph
v8
libc
JS
built ins

Flame Graph Interpretation
a()
b() h()
c()
d()
e() f()
g()
i()

Top edge shows who is running on-CPU, 
and how much (width)
a()
b() h()
c()
d()
e() f()
g()
i()

Top-down shows ancestry
e.g., from g():
h()
d()
e()
i()
a()
b()
c()
f()
g()

a()
b() h()
c()
d()
e() f()
g()
i()
Widths are proportional to presence in samples
e.g., comparing b() to h() (incl. children)

function merge(object) {
var args = arguments,
length = 2;
...

Flame Graphs
Helps you ﬁnd 1 LoC out of 6 Million

Results
❖ Dramatically reduced request latency
❖ Reduced CPU utilization
❖ Increased throughput

Runtime Performance Technique
❖ Sample stack traces via perf(1)
❖ Visualize code distribution with CPU ﬂame graphs
❖ Identify candidate code paths for performance
improvement
❖ Repeat

- Chaﬁn, R. "Pioneer F & G Telemetry and Command Processor Core Dump
Program." JPL Technical Report XVI, no. 32-1526 (1971): 174.
“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.”

Core Dumps — A Brief History
❖ Magnetic core memory
❖ Dump out the contents of
“core” memory for debugging
❖ “Core dump” was born
❖ 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
Take core
dump
Restart app
Load core
dump
elsewhere
Engineer
Fix
Debug
Continue
serving
trafﬁc

Configure Node to Dump Core on Error
!"[0] <> node --abort_on_uncaught_exception throw.js
Uncaught Error
FROM
Object.<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

Node Post Mortem Tooling
❖ Netﬂix uses Linux in Prod
❖ Linux — Work in progress
❖ https://github.com/tjfontaine/lldb-v8
❖ https://github.com/indutny/llnode
❖ Solaris — Full featured, compatible with Linux cores
❖ https://github.com/joyent/mdb_v8

Socks & Duct Tape: Setup a Debug Solaris Instance
EC2: http://omnios.omniti.com/wiki.php/
Installation#IntheCloud
VM: http://omnios.omniti.com/wiki.php/
Installation#Quickstart

Post Mortem Methodology
❖ Where: Inspect stack trace
❖ Why: Inspect heap and stack variable state

mdb(1) JS commands
❖ ::help <cmd>
❖ ::jsstack
❖ ::jsprint
❖ ::jssource
❖ ::jsconstructor
❖ ::ﬁndjsobjects
❖ ::jsfunctions

Load the Core Dump
# mdb ./node-v4.2.2-linux/node-v4.2.2-linux-x64/bin/node ./core.7186
> ::load ./mdb_v8_amd64.so
mdb_v8 version: 1.1.1 (release, from 28cedf2)
V8 version: 143.156.132.195
Autoconfigured V8 support from target
C++ symbol demangling enabled
linux node binary core dumpload mdb_v8 module

::jsstack
> ::jsstack
js: test
js: storeHeader
js: <anonymous> (as OutgoingMessage._storeHeader)
js: <anonymous> (as ServerResponse.writeHead)
js: restifyWriteHead
js: _cb
js: send
js: <anonymous> (as <anon>)
js: <anonymous> (as ReactRenderer._renderLayout)
js: <anonymous> (as dispatchHandler)
js: runHooks
js: runTransitionToHooks
js: <anonymous> (as assign.to)
js: runHooks
js: runTransitionFromHooks
js: <anonymous> (as assign.from)
js: <anonymous> (as React.createClass.statics.dispatch)
native: _ZN2v88internalL6InvokeEbNS0_6HandleINS0_10JSFunctionEEENS1_INS0...
native: v8::internal::Execution::Call+0xc8
native: v8::internal::Runtime_Apply+0x1ce
frame type
func name

Always name your functions!
var foo = function foo() {};
Foo.prototype.bar = function bar() {};
foo(function bar() {});

::jsstack -v Frame Source
> ::jsstack -v
js: storeHeader
file: http.js
posn: position 18774
this: 2ad561306c91 (<unknown>)
arg1: 3bd67e0669b9 (JSObject: ServerResponse)
arg2: 3dfe966ae299 (JSObject: Object)
arg3: 34d5391d8859 (SeqAsciiString)
652
653 function storeHeader(self, state, field, value) {
654 // Protect against response splitting. The if statement is there to
655 // minimize the performance impact in the common case.
656 if (/[rn]/.test(value))
657 value = value.replace(/[rn]+[ t]*/g, '');
658
659 state.messageHeader += field + ': ' + value + CRLF;
660
661 if (connectionExpression.test(field)) {
662 state.sentConnectionHeader = true;
663 if (closeExpression.test(value)) {
664 self._last = true;
665 } else {
666 self.shouldKeepAlive = true;
667 }
668

::jsstack -vn0 Frame and Function Args
> ::jsstack -vn0
js: test
file: native regexp.js
posn: position 2677
this: 2421205bd4d9 (JSRegExp)
js: storeHeader
file: http.js
file: http.js
this: 3bd67e0669b9 (JSObject: ServerResponse)
arg1: 3dfe966ae271 (ConsString)
arg2: 3dfe966add99 (JSObject: Object)
file: /apps/node/webapp/node_modules/restify/lib/response.js
posn: position 6964
(1 internal frame elided)
js: _cb
Func Name
JS File
Line #
Func Args

::jsstack Function Args
> ::jsstack -vn0
js: test
file: native regexp.js
posn: position 2677
this: 2421205bd4d9 (JSRegExp)
js: storeHeader
file: http.js
file: http.js
arg1: 3dfe966ae271 (ConsString)
arg2: 3dfe966add99 (JSObject: Object)
file: /apps/node/webapp/node_modules/restify/lib/response.js
posn: position 6964
(1 internal frame elided)
js: _cb
Memory Address of Var Var Type

::jsprint Print JS Objects
> 3bd67e0669b9::jsprint
{
"_time": 1437690472539,
"_headers": {
"content-type": "text/html",
"req_id": "5b7f18f2-7f12-4c68-b07f-3cd75698ba65",
"set-cookie": “CENSORED; Domain=.netflix.com; Expires=Fri, 24 Jul 2015 10:27:52 GMT
"x-frame-options": "DENY",
"x-ua-compatible": "IE=edge",
"x-netflix.client.instance": "i-c420596c",
},
"output": [],
"_last": false,
"_hangupClose": false,
"_hasBody": true,
"socket": {
"_connecting": false,
"_handle": [...],
"_readableState": [...],
"readable": true,
"domain": null,
"_events": [...],
"_maxListeners": 10,
"_writableState": [...],
"writable": true,
"allowHalfOpen": true,
Actual JS Object Instance

::jsconstructor Show Object Constructor
> 3bd67e0669b9::jsconstructor -v
ServerResponse (JSFunction: 2421205bced9)

::jssource Print f() Source
> 2421205bced9::jssource
file: http.js
1066 function ServerResponse(req) {
1067 OutgoingMessage.call(this);
1068
1069 if (req.method === 'HEAD') this._hasBody = false;
1070
1071 this.sendDate = true;
1072
1073 if (req.httpVersionMajor < 1 || req.httpVersionMinor < 1) {
1074 this.useChunkedEncodingByDefault = chunkExpression.test(req.headers.te);
1075 this.shouldKeepAlive = false;
1076 }
1077 }
1078 util.inherits(ServerResponse, OutgoingMessage);

Core Dump === Complete Process State

gcore(1) GNU Tools gcore(1)
NAME
gcore - Generate a core file for a running process
SYNOPSIS
gcore [-o filename] pid

Take a Core Dump!
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.6
Saved corefile core.5602

::findjsobjects
NAME
findjsobjects - find JavaScript objects
SYNOPSIS
[ addr ] ::findjsobjects [-vb] [-r | -c
cons | -p prop]

::findjsobjects Find ALL JS Objects on Heap
> ::findjsobjects
OBJECT #OBJECTS #PROPS CONSTRUCTOR: PROPS
...
3dfe97453121 18 6721 Array
157a020e01 1304 101 <anonymous> (as Constructor): ...
8f1a53211 13879 12 ReactDOMComponent: _tag, tagName,
props, ...
8f1a05691 85776 2 Array
3dfe97451a99 36 5589 Array
23e5d7d44351 1 218020 Object: .2f5hpw2hgjk.1.0.3, ...
8f1a05f31 40533 6 <anonymous> (as ReactElement): type, ...
8f1a04da1 252133 1 Array
8f1a04dc1 125869 7 Array
8f1a04f01 114914 8 Array
8f1a04d39 230924 7 Module: id, exports, parent,
filename, ...

Memory Leak Strategy
❖ Look at objects on heap for suspicious objects
❖ Take successive core dumps and compare object counts
❖ Growing object counts are likely leaking
❖ Inspect object for more context
❖ Walk reverse references to ﬁnd root object

Look at Object Delta Between
Successive Core Dumps

Uptime = 45mins
> ::findjsobjects
...
8f1a04d39 230924 7 Module: id, exports,
parent, filename, ...

Uptime = 90 mins
> ::findjsobjects
...

Representative Object
> ::findjsobjects
...
Representative Object, 1 of 323454

Look Closer
> 8f1a04d39::jsprint
{
"id": "/apps/node/webapp/ui/js/pages/akiraClient.js",
"exports": {},
"parent": {
"id": "/apps/node/webapp/middleware/autoClientStrings.js",
"exports": function autoExposeClientStrings,
"parent": [...],
"filename": "/apps/node/webapp/middleware/
autoClientStrings.js",
"loaded": true,
"children": [...],
"paths": [...],
},
"filename": "/apps/node/webapp/ui/js/pages/akiraClient.js",

Use ::findjsobjects to Find All “Module” Objects
> 8f1a04d39::findjsobjects
8f1a04d39
3fd996bffb39
3fd996bfcff1
3fd996bfbac1
3fd996bf8a19
3fd996bf7949
3fd996bf3ce9
3fd996bf0f19
3fd996bead71
3fd996bea821
3fd996bea001
3fd996be92b1
3fd996be73d1
3fd996be58d1
3fd996bd88b1
3fd996bcb459
3fd996bcaa41
3fd996bc7009

Custom Querying With Pipes and Unix Tools
8f1a04d39::findjsobjects | ::jsprint ! grep filename | sort | uniq -c

Results
...
1 "filename": "/apps/node/webapp/ui/js/akira/
components/messaging/paymentHold.js",
2 "filename": "/apps/node/webapp/ui/js/common/
commonCore.js",
playPrediction/playPrediction.js",
presentationTracking/presentationTracking.js",
111061 "filename": “/apps/node/webapp/ui/js/common/
playPrediction/playPrediction.js",
7103 "filename": “/apps/node/webapp/ui/js/pages/
reactClientRender.js",
111061 "filename": “/apps/node/webapp/ui/js/pages/
akiraClient.js",
118257 "filename": “/apps/node/webapp/middleware/
autoClientStrings.js",
... Client Side Modules

What’s holding on to these
modules?

Walk Reverse Refs with ::findjsobjects -r
> 8f1a04d39::findjsobjects -r
8f1a04d39 referred to by 14fd6c5b13c1.parent

Root Object
> 1f313791bb41::jsprint
[
{
"exports": [...],
"parent": [...],
"loaded": false,
"children": [...],
"paths": [...],
},
{
"exports": [...],
"parent": [...],
"loaded": false,
"children": [...],
"paths": [...],
},
{
"exports": [...],
"parent": [...],

Spot the Leak
var cache = {};
function checkCache(someModule) {
var mod = cache[someModule];
if (!mod) {
try {
mod = require(someModule);
cache[someModule] = mod;
return mod;
} catch (e) {
return {};
}
}
return mod;
}
Module could be client only, must catch
Should cache the
fact we caught an exception here

Root Cause
❖ Node caches metadata for each module
❖ If require process throws an exception, the module
metadata is leaked (bug?)
❖ Client side module meant we were throwing during
every request, and not caching the fact we tried to
require it
❖ Each request leaks 3+ module metadata objects

Memory Leaks
❖ Take successive core dumps (gcore(1))
❖ Compare object counts (::findjsobjects)
❖ Growing objects are likely leaking
❖ Inspect object for more context (::jsprint)
❖ Walk reverse references to find root obj (::findjsobjects -
r)

Post Mortem Debugging is Critical to
Large Scale Prod Node Deployments

More State than Just Logs
❖ Detailed stack trace (::jsstack)
❖ Function args for each frame (::jsstack -vn0)
❖ Get state of any object and its provenance
(::jsprint, ::jsconstructor)
❖ Get source code of any function (::jssource)
❖ Find arbitrary JS objects (::ﬁndjsobjects)
❖ Unmodiﬁed Node binary!

Production Failures are Inevitable

Production Debugging
❖ Runtime Performance
❖ CPU proﬁling/ﬂame graphs
❖ Runtime Crashes
❖ Inspect program state with core dumps and mdb
❖ Memory leaks
❖ Analyze objects and references with core dumps and
mdb

Epilogue — State of Tooling
❖ Join Working Group https://github.com/nodejs/post-
mortem
❖ Help make mdb_v8 cross platform https://github.com/
joyent/mdb_v8
❖ Contribute to https://github.com/tjfontaine/lldb-v8
and https://github.com/indutny/llnode

Acknowledgements
❖ mdb_v8
❖ Dave Pacheco, TJ Fontaine, Julien Gilli, Bryan Cantrill
❖ CPU Proﬁling/Flamegraphs
❖ Brendan Gregg, Google V8 team, Ali Ijaz Sheikh
❖ Linux Perf
❖ Ingo Molnar, Arnaldo Carvalho de Melo, Namhyung Kim, Jiri Olsa, Peter Zijlstra
❖ lldb-v8
❖ TJ Fontaine
❖ llnode
❖ Fedor Indutny

THANKS
❖ Questions? We’re Hiring!
❖ yunong@netﬂix.com
❖ @yunongx

Citations
❖ Slides 29-32 used with permission from “Java Mixed-
Mode Flame Graphs”, Brendan Gregg, Oct 2015
❖ Slide 26 used with permission from http://
www.brendangregg.com/FlameGraphs/
cpuﬂamegraphs.html

Node Interactive Debugging Node.js In Production

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