Kernel Recipes 2017: Using Linux perf at Netflix

Using Linux perf
at Netflix
Brendan Gregg
Senior Performance Architect
Sep 2017

Case Study: ZFS is ea/ng my CPU
•  Easy to debug using Ne9lix Vector & ﬂame graphs
•  How I expected it to look:

Case Study: ZFS is ea/ng my CPU (cont.)
•  How it really looked:
Applica/on (truncated)
38% kernel /me (why?)

Case Study: ZFS is ea/ng my CPU (cont.)
•  ZFS ARC (adap/ve replacement cache) reclaim.
•  But… ZFS is not in use. No pools, datasets, or ARC buﬀers.
•  CPU /me is in random entropy, picking which (empty) list to evict.

Bug: hUps://github.com/zfsonlinux/zfs/issues/6531
Zoomed:

Agenda
1.  Why Ne9lix Needs Linux Proﬁling
2.  perf Basics
3.  CPU Proﬁling & Gotchas
–  Stacks (gcc, Java)
–  Symbols (Node.js, Java)
–  Guest PMCs
–  PEBS
–  Overheads
4.  perf Advanced

1. Why Ne)lix Needs Linux Proﬁling

Understand CPU usage quickly and completely

Quickly
Flame Graphs
Heat Maps
…
Eg, Ne9lix Vector (self-service UI):

Java
Kernel
(C) JVM
(C++)
Completely
User
(C)
CPU Flame Graph

Why Linux perf?
•  Available
–  Linux, open source
•  Low overhead
–  Tunable sampling, ring buﬀers
•  Accurate
–  Applica/on-basic samplers don't know what's really RUNNING; eg, Java and epoll
•  No blind spots
–  See user, library, kernel with CPU sampling
–  With some work: hardirqs & SMI as well
•  No sample skew
–  Unlike Java safety point skew

Why is this so important
•  We typically scale microservices based on %CPU
–  Small %CPU improvements can mean big $avings
•  CPU profiling is used by many ac/vi/es
–  Explaining regressions in new sooware versions
–  Incident response
–  3rd party sooware evalua/ons
–  Iden/fy performance tuning targets
–  Part of CPU workload characteriza/on
•  perf does lots more, but we spend ~95% of our /me looking
at CPU profiles, and 5% on everything else
–  With new BPF capabili/es (off-CPU analysis), that might go from 95 to 90%

CPU profiling should be easy, but…
JIT runtimes
no frame pointers
no debuginfo
stale symbol maps
container namespaces
…

perf (aka "perf_events")
•  The official Linux profiler
–  In the linux-tools-common package
–  Source code & docs in Linux: tools/perf
•  Supports many profiling/tracing features:
–  CPU Performance Monitoring Counters (PMCs)
–  Sta/cally defined tracepoints
–  User and kernel dynamic tracing
–  Kernel line and local variable tracing
–  Efficient in-kernel counts and filters
–  Stack tracing, libunwind
–  Code annota/on
•  Some bugs in the past; has been stable for us
perf_events
ponycorn

A Mul/tool of Subcommands
# perf
usage: perf [--version] [--help] [OPTIONS] COMMAND [ARGS]
The most commonly used perf commands are:
annotate Read perf.data (created by perf record) and display annotated code
archive Create archive with object files with build-ids found in perf.data file
bench General framework for benchmark suites
buildid-cache Manage build-id cache.
buildid-list List the buildids in a perf.data file
c2c Shared Data C2C/HITM Analyzer.
config Get and set variables in a configuration file.
data Data file related processing
diff Read perf.data files and display the differential profile
evlist List the event names in a perf.data file
ftrace simple wrapper for kernel's ftrace functionality
inject Filter to augment the events stream with additional information
kallsyms Searches running kernel for symbols
kmem Tool to trace/measure kernel memory properties
kvm Tool to trace/measure kvm guest os
list List all symbolic event types
lock Analyze lock events
mem Profile memory accesses
record Run a command and record its profile into perf.data
report Read perf.data (created by perf record) and display the profile
sched Tool to trace/measure scheduler properties (latencies)
script Read perf.data (created by perf record) and display trace output
stat Run a command and gather performance counter statistics
test Runs sanity tests.
timechart Tool to visualize total system behavior during a workload
top System profiling tool.
probe Define new dynamic tracepoints
trace strace inspired tool
See 'perf help COMMAND' for more information on a specific command. from Linux 4.13

perf Basic Workflow
1.  list -> find events
2.  stat -> count them
3.  record-> write event data to file
4.  report -> browse summary
5.  script -> event dump for post processing

Basic Workﬂow Example
# perf list sched:*
[…]
sched:sched_process_exec [Tracepoint event]
[…]
# perf stat -e sched:sched_process_exec -a -- sleep 10
Performance counter stats for 'system wide':
19 sched:sched_process_exec
10.001327817 seconds time elapsed
# perf record -e sched:sched_process_exec -a -g -- sleep 10
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.212 MB perf.data (21 samples) ]
# perf report -n --stdio
# Children Self Samples Trace output
# ........ ........ ............ .................................................
4.76% 4.76% 1 filename=/bin/bash pid=7732 old_pid=7732
|
---_start
return_from_SYSCALL_64
do_syscall_64
sys_execve
do_execveat_common.isra.35
[…]
# perf script
sleep 7729 [003] 632804.699184: sched:sched_process_exec: filename=/bin/sleep pid=7729 old_pid=7729
44b97e do_execveat_common.isra.35 (/lib/modules/4.13.0-rc1-virtual/build/vmlinux)
44bc01 sys_execve (/lib/modules/4.13.0-rc1-virtual/build/vmlinux)
203acb do_syscall_64 (/lib/modules/4.13.0-rc1-virtual/build/vmlinux)
acd02b return_from_SYSCALL_64 (/lib/modules/4.13.0-rc1-virtual/build/vmlinux)
c30 _start (/lib/x86_64-linux-gnu/ld-2.23.so)
[…]
1.  found an event of interest

2.  19 per 10 sec is a very low
rate, so safe to record

3.  21 samples captured

4.  summary style may be
suﬃcient, or,
5.  script output in /me order

perf stat/record Format
•  These have three main parts: ac/on, event, scope.
•  e.g., proﬁling on-CPU stack traces:
perf record -F 99 -a -g -- sleep 10
Event: 99 Hertz
Scope: all CPUs
AcCon: record stack traces
Note: sleep 10 is a dummy command to set the dura/on

perf Ac/ons
•  Count events (perf stat …)
–  Uses an efficient in-kernel counter, and prints the results
•  Sample events (perf record …)
–  Records details of every event to a dump file (perf.data)
•  Timestamp, CPU, PID, instruc/on pointer, …
–  This incurs higher overhead, rela/ve to the rate of events
–  Include the call graph (stack trace) using -g
•  Other ac/ons include:
–  List events (perf list)
–  Report from a perf.data file (perf report)
–  Dump a perf.data file as text (perf script)
–  top style profiling (perf top)

perf Events
•  Custom /mers
–  e.g., 99 Hertz (samples per second)
•  Hardware events
–  CPU Performance Monitoring Counters (PMCs)
•  Tracepoints
–  Sta/cally deﬁned in sooware
•  Dynamic tracing
–  Created using uprobes (user) or kprobes (kernel)
–  Can do kernel line tracing with local variables (needs kernel debuginfo)

perf Events: List
# perf list
List of pre-defined events (to be used in -e):
cpu-cycles OR cycles [Hardware event]
instructions [Hardware event]
cache-references [Hardware event]
cache-misses [Hardware event]
branch-instructions OR branches [Hardware event]
branch-misses [Hardware event]
bus-cycles [Hardware event]
stalled-cycles-frontend OR idle-cycles-frontend [Hardware event]
stalled-cycles-backend OR idle-cycles-backend [Hardware event]
[…]
cpu-clock [Software event]
task-clock [Software event]
page-faults OR faults [Software event]
context-switches OR cs [Software event]
cpu-migrations OR migrations [Software event]
[…]
L1-dcache-loads [Hardware cache event]
L1-dcache-load-misses [Hardware cache event]
L1-dcache-stores [Hardware cache event]
[…]
skb:kfree_skb [Tracepoint event]
skb:consume_skb [Tracepoint event]
skb:skb_copy_datagram_iovec [Tracepoint event]
net:net_dev_xmit [Tracepoint event]
net:net_dev_queue [Tracepoint event]
net:netif_receive_skb [Tracepoint event]
net:netif_rx [Tracepoint event]
[…]

perf Scope
•  System-wide: all CPUs (-a)
•  Target PID (-p PID)
•  Target command (…)
•  Speciﬁc CPUs (-c …)
•  User-level only (<event>:u)
•  Kernel-level only (<event>:k)
•  A custom ﬁlter to match variables (--filter …)
•  This cgroup (container) only (--cgroup …)

One-Liners: Lis/ng Events
# Listing all currently known events:
perf list
# Searching for "sched" tracepoints:
perf list | grep sched
# Listing sched tracepoints:
perf list 'sched:*'
Dozens of perf one-liners:
hUp://www.brendangregg.com/perf.html#OneLiners

One-Liners: Coun/ng Events
# CPU counter statistics for the specified command:
perf stat command
# CPU counter statistics for the entire system, for 5 seconds:
perf stat -a sleep 5
# Detailed CPU counter statistics for the specified PID, until Ctrl-C:
perf stat -dp PID
# Various CPU last level cache statistics for the specified command:
perf stat -e LLC-loads,LLC-load-misses,LLC-stores,LLC-prefetches command
# Count system calls for the specified PID, until Ctrl-C:
perf stat -e 'syscalls:sys_enter_*' -p PID
# Count block device I/O events for the entire system, for 10 seconds:
perf stat -e 'block:*' -a sleep 10
# Show system calls by process, refreshing every 2 seconds:
perf top -e raw_syscalls:sys_enter -ns comm

One-Liners: Proﬁling Events
# Sample on-CPU functions for the specified command, at 99 Hertz:
perf record -F 99 command
# Sample CPU stack traces for the specified PID, at 99 Hertz, for 10 seconds:
perf record -F 99 -p PID -g -- sleep 10
# Sample CPU stack traces for the entire system, at 99 Hertz, for 10 seconds:
perf record -F 99 -ag -- sleep 10
# Sample CPU stacks, once every 10,000 Level 1 data cache misses, for 5 secs:
perf record -e L1-dcache-load-misses -c 10000 -ag -- sleep 5
# Sample CPU stack traces, once every 100 last level cache misses, for 5 secs:
perf record -e LLC-load-misses -c 100 -ag -- sleep 5
# Sample on-CPU kernel instructions, for 5 seconds:
perf record -e cycles:k -a -- sleep 5
# Sample on-CPU user instructions, for 5 seconds:
perf record -e cycles:u -a -- sleep 5

One-Liners: Repor/ng
# Show perf.data in an ncurses browser (TUI) if possible:
perf report
# Show perf.data with a column for sample count:
perf report -n
# Show perf.data as a text report, with data coalesced and percentages:
perf report --stdio
# List all raw events from perf.data:
perf script
# List all raw events from perf.data, with customized fields:
perf script -f comm,tid,pid,time,cpu,event,ip,sym,dso
# Dump raw contents from perf.data as hex (for debugging):
perf script -D
# Disassemble and annotate instructions with percentages (needs debuginfo):
perf annotate --stdio

CPU Profiling
•  Record stacks at a /med interval: simple and effec/ve
–  Pros: Low (determinis/c) overhead
–  Cons: Coarse accuracy, but usually sufficient
A
B
block interrupt
on-CPU off-CPU
A
B
A A
B
A
syscall
/me
stack
samples: A

perf Record
# perf record -F 99 -ag -- sleep 30
[ perf record: Woken up 9 times to write data ]
[ perf record: Captured and wrote 2.745 MB perf.data (~119930 samples) ]
# perf report -n --stdio
1.40% 162 java [kernel.kallsyms] [k] _raw_spin_lock
|
--- _raw_spin_lock
|
|--63.21%-- try_to_wake_up
| |
| |--63.91%-- default_wake_function
| | |
| | |--56.11%-- __wake_up_common
| | | __wake_up_locked
| | | ep_poll_callback
| | | __wake_up_common
| | | __wake_up_sync_key
| | | |
| | | |--59.19%-- sock_def_readable
[…78,000 lines truncated…]
Sampling full
stack traces
at 99 Hertz

perf Repor/ng
•  perf report summarizes by combining common paths
•  Previous output truncated 78,000 lines of summary
•  The following is what a mere 8,000 lines looks like…

Flame Graphs
•  Flame Graphs:
–  x-axis: alphabe/cal stack sort, to maximize merging
–  y-axis: stack depth
–  color: random, or hue can be a dimension
•  e.g., sooware type, or difference between two profiles for
non-regression tes/ng ("differen/al flame graphs")
–  interpreta/on: top edge is on-CPU, beneath it is ancestry
•  Just a Perl program to convert perf stacks into SVG
–  Includes JavaScript: open in a browser for interac/vity
•  Easy to get working hUp://www.brendangregg.com/FlameGraphs/cpuflamegraphs.html
git clone --depth 1 https://github.com/brendangregg/FlameGraph
cd FlameGraph
perf record -F 99 -a –g -- sleep 30
perf script | ./stackcollapse-perf.pl | ./flamegraph.pl > perf.svg

ﬂamegraph.pl Op/ons
$ flamegraph.pl --help
USAGE: flamegraph.pl [options] infile > outfile.svg
--title TEXT # change title text
--subtitle TEXT # second level title (optional)
--width NUM # width of image (default 1200)
--height NUM # height of each frame (default 16)
--minwidth NUM # omit smaller functions (default 0.1 pixels)
--fonttype FONT # font type (default "Verdana")
--fontsize NUM # font size (default 12)
--countname TEXT # count type label (default "samples")
--nametype TEXT # name type label (default "Function:")
--colors PALETTE # set color palette. choices are: hot (default), mem,
# io, wakeup, chain, java, js, perl, red, green, blue,
# aqua, yellow, purple, orange
--hash # colors are keyed by function name hash
--cp # use consistent palette (palette.map)
--reverse # generate stack-reversed flame graph
--inverted # icicle graph
--negate # switch differential hues (blue<->red)
--notes TEXT # add notes comment in SVG (for debugging)
--help # this message
eg,
flamegraph.pl --title="Flame Graph: malloc()" trace.txt > graph.svg

perf Flame Graph Workflow (Linux 2.6+)
perf stat perf record
perf report perf script
count events capture stacks
text UI dump profile
stackcollapse-perf.pl
perf.data
flame graph
visualiza/on
perf list
list events
Typical
Workflow
flamegraph.pl

perf Flame Graph Workflow (Linux 4.5+)
perf report perf report
-g folded
text UI summary
awk
flame graph
visualiza/on
flamegraph.pl
perf script
dump
perf stat perf record
count events capture stacks
perf.data
perf list
list events
Typical
Workflow

Flame Graph Op/miza/ons
Linux 2.6
perf record
perf script
capture stacks
write text
stackcollapse-perf.pl
flamegraph.pl
perf.data
write samples
reads samples
folded output
perf record
perf report
–g folded
capture stacks
folded report
awk
flamegraph.pl
perf.data
write samples
reads samples
folded output
Linux 4.5
count stacks (BPF)
folded
output
flamegraph.pl
profile.py
Linux 4.9
not perf

When we've tried to use perf
•  Stacks don't work (missing)
•  Symbols don't work (hex numbers)
•  Instruc/on proﬁling looks bogus
•  PMCs don't work in VM guests
•  Container break things
•  Overhead is too high

How to really get started
1.  Get "perf" to work
2.  Get stack walking to work
3.  Fix symbol transla/on
4.  Get IPC to work
5.  Test perf under load
Install perf-tools-common and
perf-tools-`uname -r` packages;
Or compile in the Linux source:
tools/perf
The "gotchas"…

Gotcha #1 Broken Stacks
1.  Take a CPU proﬁle
2.  Run perf report
3.  If stacks are ooen < 3 frames, or don't reach "thread start" or
"main", they are probably broken. Fix them.
perf record -F 99 -a –g -- sleep 30
perf report -n --stdio

Iden/fying Broken Stacks
|--11.65%-- 0x40a447
| 0x40659a
| 0x408dd8
| 0x408ed1
| 0x402689
| 0x7fa1cd08aec5
|
|--1.33%-- 0x40a4a1
| |
| |--60.01%-- 0x40659a
| | 0x408dd8
| | 0x408ed1
| | 0x402689
| | 0x7fa1cd08aec5
78.50% 409 sed libc-2.19.so [.] 0x00000000000dd7d4
|
|--3.65%-- 0x7f2516d5d10d
|
|--2.19%-- 0x7f2516d0332f
|
|--1.22%-- 0x7f2516cffbd2
|
|--1.22%-- 0x7f2516d5d5ad
broken
probably not broken
missing symbols, but
that's another problem

Broken Stacks Flame Graph
Broken Java stacks
(missing frame pointer)
Java == green
system == red
C++ == yellow

Fixing Broken Stacks
•  Either:
•  Fix frame pointer-based stack walking (the default)
–  Pros: simple, supports any system stack walker
–  Cons: might cost a liUle extra CPU to make available
•  Use libunwind and DWARF: perf record -g dwarf
–  Pros: more debug info
–  Cons: not on older kernels, and inflates instance size
–  … there's also ORC on the latest kernel
•  Applica/on support
–  hUps://github.com/jvm-profiling-tools/async-profiler
•  Our current preference is (A), but (C) is also promising
–  So how do we fix the frame pointer…

gcc -fno-omit-frame-pointer
•  Once upon a time, x86 had fewer registers, and the frame
pointer register was reused for general purpose to improve
performance. This breaks system stack walking.
•  gcc provides -fno-omit-frame-pointer to ﬁx this
–  Please make this the default in gcc!

Java -XX:+PreserveFramePointer
•  I hacked frame pointers in the JVM (JDK-8068945) and Oracle rewrote
it as -XX:+PreserveFramePointer. Lets perf do FP stack walks of Java.

•  Costs some overhead to use. Usually <1%. Rare cases 10%.
--- openjdk8clean/hotspot/src/cpu/x86/vm/macroAssembler_x86.cpp 2014-03-04…
+++ openjdk8/hotspot/src/cpu/x86/vm/macroAssembler_x86.cpp 2014-11-07 …
@@ -5236,6 +5236,7 @@
// We always push rbp, so that on return to interpreter rbp, will be
// restored correctly and we can correct the stack.
push(rbp);
+ mov(rbp, rsp);
// Remove word for ebp
framesize -= wordSize;
--- openjdk8clean/hotspot/src/cpu/x86/vm/c1_MacroAssembler_x86.cpp …
+++ openjdk8/hotspot/src/cpu/x86/vm/c1_MacroAssembler_x86.cpp …
[...]
Involved changes like this:
ﬁxing x86-64 func/on
prologues

Broken Java Stacks
•  Check with "perf script" to see
stack samples
•  These are 1 or 2 levels deep (junk
values)
# perf script
[…]
java 4579 cpu-clock:
ffffffff8172adff tracesys ([kernel.kallsyms])
7f4183bad7ce pthread_cond_timedwait@@GLIBC_2…
7f417908c10b [unknown] (/tmp/perf-4458.map)
7f4179101c97 [unknown] (/tmp/perf-4458.map)
7f41792fc65f [unknown] (/tmp/perf-4458.map)
a2d53351ff7da603 [unknown] ([unknown])
7f4179349aec [unknown] (/tmp/perf-4458.map)
7f4179101d0f [unknown] (/tmp/perf-4458.map)
[…]

Fixed Java Stacks
•  With -XX:+PreserveFramePointer
stacks are full, and go all the way to
start_thread()
•  This is what the CPUs are really
running: inlined frames are not
present
# perf script
[…]
7fff76f2dce1 [unknown] ([vdso])
7fd3173f7a93 os::javaTimeMillis() (/usr/lib/jvm…
7fd301861e46 [unknown] (/tmp/perf-8131.map)
7fd30184def8 [unknown] (/tmp/perf-8131.map)
7fd30174f544 [unknown] (/tmp/perf-8131.map)
7fd30175d3a8 [unknown] (/tmp/perf-8131.map)
7fd30166d51c [unknown] (/tmp/perf-8131.map)
7fd3016c2280 [unknown] (/tmp/perf-8131.map)
7fd301b02ec0 [unknown] (/tmp/perf-8131.map)
7fd3016ece04 [unknown] (/tmp/perf-8131.map)
7fd30177783c [unknown] (/tmp/perf-8131.map)
7fd301600aa8 [unknown] (/tmp/perf-8131.map)
7fd301a4484c [unknown] (/tmp/perf-8131.map)
7fd301007325 [unknown] (/tmp/perf-8131.map)
7fd301007325 [unknown] (/tmp/perf-8131.map)
7fd3171df76a JavaCalls::call_helper(JavaValue*,…
7fd3171dce44 JavaCalls::call_virtual(JavaValue*…
7fd3171dd43a JavaCalls::call_virtual(JavaValue*…
7fd31721b6ce thread_entry(JavaThread*, Thread*)…
7fd3175389e0 JavaThread::thread_main_inner() (/…
7fd317538cb2 JavaThread::run() (/usr/lib/jvm/nf…
7fd3173f6f52 java_start(Thread*) (/usr/lib/jvm/…
7fd317a7e182 start_thread (/lib/x86_64-linux-gn…

Fixed Stacks Flame Graph
Java
(no symbols)

Fixing Symbols
•  For installed packages:
A.  Add a -dbgsym package, if available
B.  Recompile from source
•  For JIT (Java, Node.js, …):
A.  Create a /tmp/perf-PID.map ﬁle. perf already looks for this
•  Map format is "START SIZE symbolname"
B.  Or use a symbol loggers. Eg tools/perf/jvm/.
# perf script
Failed to open /tmp/perf-8131.map, continuing without symbols
[…]
7fff76f2dce1 [unknown] ([vdso])
7fd3173f7a93 os::javaTimeMillis() (/usr/lib/jvm…
[…]

Java Symbols
•  perf-map-agent
–  Agent aUaches and writes the map file on demand (previous versions aUached on Java
start, and wrote con/nually)
–  hUps://github.com/jvm-profiling-tools/perf-map-agent
(was hUps://github.com/jrudolph/perf-map-agent)
•  Automa/on: jmaps
–  We use scripts to find Java processes and dump their map files, paying aUen/on to file
ownership etc
–  hUps://github.com/brendangregg/FlameGraph/blob/master/jmaps
–  Needs to run as close as possible to the profile, to minimize symbol churn
# perf record -F 99 -a -g -- sleep 30; jmaps

Java
Kernel
(C)
JVM
(C++)
Java Flame Graph: Stacks & Symbols
User
(C)
flamegraph.pl --color=java

Java: Inlining
A.  Disabling inlining:
–  -XX:-Inline
–  Many more Java frames
–  80% slower (in this case)
–  May not be necessary: inlined ﬂame
graphs ooen make enough sense
–  Or tune -XX:MaxInlineSize and -
XX:InlineSmallCode to reveal more
frames, without cos/ng much perf: can
even go faster!
B.  Symbol agents can uninline
–  perf-map-agent unfoldall
–  We some/mes need and use this
No inlining

Node.js: Stacks & Symbols
•  Frame pointer stacks work
•  Symbols currently via a logger
–  --perf-basic-prof: everything. We found it can log over 1 Gbyte/day.
–  --perf-basic-prof-only-functions: tries to only log symbols we care about.
•  perf may not use the most recent symbol in the log
–  We /dy logs before using them:
hUps://raw.githubusercontent.com/brendangregg/Misc/master/perf_events/
perfmap/dy.pl
•  Future v8's may support on-demand symbol dumps

Gotcha #3 Instruc/on Proﬁling
# perf annotate -i perf.data.noplooper --stdio
Percent | Source code & Disassembly of noplooper
--------------------------------------------------------
: Disassembly of section .text:
:
: 00000000004004ed <main>:
0.00 : 4004ed: push %rbp
0.00 : 4004ee: mov %rsp,%rbp
20.86 : 4004f1: nop
0.00 : 4004f2: nop
0.00 : 4004f3: nop
0.00 : 4004f4: nop
19.84 : 4004f5: nop
0.00 : 4004f6: nop
0.00 : 4004f7: nop
0.00 : 4004f8: nop
18.73 : 4004f9: nop
0.00 : 4004fa: nop
0.00 : 4004fb: nop
0.00 : 4004fc: nop
19.08 : 4004fd: nop
0.00 : 4004fe: nop
0.00 : 4004ff: nop
0.00 : 400500: nop
21.49 : 400501: jmp 4004f1 <main+0x4>
16 NOPs in a loop

Let's proﬁle instruc/ons
to see which are hot!

(have I lost my mind?)

Instruc/on Proﬁling
•  Even distribu/on (A)? Or something else?
(A) (B)
(C) (D)

Instruc/on Proﬁling
# perf annotate -i perf.data.noplooper --stdio
Percent | Source code & Disassembly of noplooper
--------------------------------------------------------
: Disassembly of section .text:
:
: 00000000004004ed <main>:
0.00 : 4004ed: push %rbp
0.00 : 4004ee: mov %rsp,%rbp
20.86 : 4004f1: nop
0.00 : 4004f2: nop
0.00 : 4004f3: nop
0.00 : 4004f4: nop
19.84 : 4004f5: nop
0.00 : 4004f6: nop
0.00 : 4004f7: nop
0.00 : 4004f8: nop
18.73 : 4004f9: nop
0.00 : 4004fa: nop
0.00 : 4004fb: nop
0.00 : 4004fc: nop
19.08 : 4004fd: nop
0.00 : 4004fe: nop
0.00 : 4004ff: nop
0.00 : 400500: nop
21.49 : 400501: jmp 4004f1 <main+0x4>
Go home instruc/on pointer, you're drunk

PEBS
•  I believe this is due to parallel and out-of-order execu/on of
micro-ops: the sampled IP is the resump/on instruc/on, not
what is currently execu/ng. And skid.
•  PEBS may help: Intel's Precise Event Based Sampling
•  perf_events has support:
–  perf record -e cycles:pp
–  The 'p' can be speciﬁed mul/ple /mes:
•  0 - SAMPLE_IP can have arbitrary skid
•  1 - SAMPLE_IP must have constant skid
•  2 - SAMPLE_IP requested to have 0 skid
•  3 - SAMPLE_IP must have 0 skid
–  … from tools/perf/Documenta/on/perf-list.txt

Gotcha #4 VM Guests
•  Using PMCs from most VM guests:
# perf stat -a -d sleep 5
Performance counter stats for 'system wide':
10003.718595 task-clock (msec) # 2.000 CPUs utilized [100.00%]
323 context-switches # 0.032 K/sec [100.00%]
17 cpu-migrations # 0.002 K/sec [100.00%]
233 page-faults # 0.023 K/sec
<not supported> cycles
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
<not supported> instructions
<not supported> branches
<not supported> branch-misses
<not supported> L1-dcache-loads
<not supported> L1-dcache-load-misses
<not supported> LLC-loads
<not supported> LLC-load-misses
5.001607197 seconds time elapsed

VM Guest PMCs
•  Without PMCs, %CPU is ambiguous. We need IPC.
–  Can't measure instruc/ons per cycle (IPC), cache hits/misses, MMU/TLB events, etc.
•  Is ﬁxable: eg, Xen can enable PMCs (vpmu boot op/on)
–  I added vpmu support for subsets, eg, vpmu=arch for Intel architectural set (7 PMCs only)
–  hUp://www.brendangregg.com/blog/2017-05-04/the-pmcs-of-ec2.html
–  Now available on the largest AWS EC2 instance types
architectural
set

VM Guest MSRs
•  Model Speciﬁc Registers (MSRs) may be exposed when PMCs are not
•  BeUer than nothing. Can solve some issues.

–  showboost is from my msr-cloud-tools collec/on (on github)
# ./showboost
CPU MHz : 2500
Turbo MHz : 2900 (10 active)
Turbo Ratio : 116% (10 active)
CPU 0 summary every 5 seconds...
TIME C0_MCYC C0_ACYC UTIL RATIO MHz
17:28:03 4226511637 4902783333 33% 116% 2900
17:28:08 4397892841 5101713941 35% 116% 2900
17:28:13 4550831380 5279462058 36% 116% 2900
17:28:18 4680962051 5429605341 37% 115% 2899
17:28:23 4782942155 5547813280 38% 115% 2899
[...]

VM Guest PEBS
•  Not possible yet in Xen
–  please ﬁx
•  DiUo for LBR, BTS, processor trace

Gotcha #5 Containers
•  perf from the host can't find symbol files in different mount
namespaces
•  We currently workaround it
–  hUp://blog.alicegoldfuss.com/making-flamegraphs-with-containerized-java/
•  Should be fixed in 4.14
–  Krister Johansen's patches

Gotcha #6 Overhead
•  Overhead is rela/ve to the rate of events instrumented
•  perf stat does in-kernel counts: rela/vely low overhead
•  perf record writes perf.data, which has slightly higher
CPU overhead, plus ﬁle system and disk I/O
•  Test before use
–  In the lab
–  Run perf stat to understand rate, before perf record
•  Also consider --filter, to ﬁlter events in-kernel

# perf record -e block:block_rq_insert -a
^C[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.172 MB perf.data (~7527 samples) ]
# perf script
[…]
java 9940 [015] 1199510.044783: block_rq_insert: 202,1 R 0 () 4783360 + 88 [java]
[…]
Tracepoints
include/trace/events/block.h: java 9940 [015] 1199510.044783: block_rq_insert: 202,1 R 0 () 4783360 + 88 [java]
DECLARE_EVENT_CLASS(block_rq,
[...]
TP_printk("%d,%d %s %u (%s) %llu + %u [%s]",
MAJOR(__entry->dev), MINOR(__entry->dev),
__entry->rwbs, __entry->bytes, __get_str(cmd),
(unsigned long long)__entry->sector,
__entry->nr_sector, __entry->comm)
format string process PID [CPU] /mestamp: eventname:
kernel source
may be the
only docs
Also see: cat /sys/kernel/debug/tracing/events/block/block_rq_insert/format

One-Liners: Sta/c Tracing
# Trace new processes, until Ctrl-C:
perf record -e sched:sched_process_exec -a
# Trace all context-switches with stack traces, for 1 second:
perf record -e context-switches –ag -- sleep 1
# Trace CPU migrations, for 10 seconds:
perf record -e migrations -a -- sleep 10
# Trace all connect()s with stack traces (outbound connections), until Ctrl-C:
perf record -e syscalls:sys_enter_connect –ag
# Trace all block device (disk I/O) requests with stack traces, until Ctrl-C:
perf record -e block:block_rq_insert -ag
# Trace all block device issues and completions (has timestamps), until Ctrl-C:
perf record -e block:block_rq_issue -e block:block_rq_complete -a
# Trace all block completions, of size at least 100 Kbytes, until Ctrl-C:
perf record -e block:block_rq_complete --filter 'nr_sector > 200'
# Trace all block completions, synchronous writes only, until Ctrl-C:
perf record -e block:block_rq_complete --filter 'rwbs == "WS"'
# Trace all block completions, all types of writes, until Ctrl-C:
perf record -e block:block_rq_complete --filter 'rwbs ~ "*W*"'
# Trace all ext4 calls, and write to a non-ext4 location, until Ctrl-C:
perf record -e 'ext4:*' -o /tmp/perf.data -a

One-Liners: Dynamic Tracing
# Add a tracepoint for the kernel tcp_sendmsg() function entry (--add optional):
perf probe --add tcp_sendmsg
# Remove the tcp_sendmsg() tracepoint (or use --del):
perf probe -d tcp_sendmsg
# Add a tracepoint for the kernel tcp_sendmsg() function return:
perf probe 'tcp_sendmsg%return'
# Show avail vars for the tcp_sendmsg(), plus external vars (needs debuginfo):
perf probe -V tcp_sendmsg --externs
# Show available line probes for tcp_sendmsg() (needs debuginfo):
perf probe -L tcp_sendmsg
# Add a tracepoint for tcp_sendmsg() line 81 with local var seglen (debuginfo):
perf probe 'tcp_sendmsg:81 seglen'
# Add a tracepoint for do_sys_open() with the filename as a string (debuginfo):
perf probe 'do_sys_open filename:string'
# Add a tracepoint for myfunc() return, and include the retval as a string:
perf probe 'myfunc%return +0($retval):string'
# Add a tracepoint for the user-level malloc() function from libc:
perf probe -x /lib64/libc.so.6 malloc
# List currently available dynamic probes:
perf probe -l

One-Liners: Advanced Dynamic Tracing
# Add a tracepoint for tcp_sendmsg(), with three entry regs (platform specific):
perf probe 'tcp_sendmsg %ax %dx %cx'
# Add a tracepoint for tcp_sendmsg(), with an alias ("bytes") for %cx register:
perf probe 'tcp_sendmsg bytes=%cx'
# Trace previously created probe when the bytes (alias) var is greater than 100:
perf record -e probe:tcp_sendmsg --filter 'bytes > 100'
# Add a tracepoint for tcp_sendmsg() return, and capture the return value:
perf probe 'tcp_sendmsg%return $retval'
# Add a tracepoint for tcp_sendmsg(), and "size" entry argument (debuginfo):
perf probe 'tcp_sendmsg size'
# Add a tracepoint for tcp_sendmsg(), with size and socket state (debuginfo):
perf probe 'tcp_sendmsg size sk->__sk_common.skc_state'
# Trace previous probe when size > 0, and state != TCP_ESTABLISHED(1) (debuginfo):
perf record -e probe:tcp_sendmsg --filter 'size > 0 && skc_state != 1' -a
•  Kernel debuginfo is an onerous requirement for the Netflix cloud
•  We can use registers instead (as above). But which registers?

The RoseUa Stone of Registers
# perf probe -nv 'tcp_sendmsg size sk->__sk_common.skc_state'
[…]
Added new event:
Writing event: p:probe/tcp_sendmsg tcp_sendmsg+0 size=%cx:u64 skc_state=+18(%si):u8
probe:tcp_sendmsg (on tcp_sendmsg with size skc_state=sk->__sk_common.skc_state)
You can now use it in all perf tools, such as:
perf record -e probe:tcp_sendmsg -aR sleep 1
One server instance with kernel debuginfo, and -nv (dry run, verbose):
# perf probe 'tcp_sendmsg+0 size=%cx:u64 skc_state=+18(%si):u8'
Failed to find path of kernel module.
Added new event:
probe:tcp_sendmsg (on tcp_sendmsg with size=%cx:u64 skc_state=+18(%si):u8)
You can now use it in all perf tools, such as:
perf record -e probe:tcp_sendmsg -aR sleep 1
All other instances (of the same kernel version):
Copy-n-paste!
Masami Hiramatsu was investigating a way to better automate this

perf Visualiza/ons: Block I/O Latency Heat Map
•  We automated this for analyzing disk I/O latency issues
hUp://www.brendangregg.com/blog/2014-07-01/perf-heat-maps.html
SSD I/O
(fast, with queueing)
HDD I/O
(random, modes)

There's s/ll a lot more to perf…
•  Using PMCs
•  perf scrip/ng interface
•  perf + eBPF
•  perf sched
•  perf /mechart
•  perf trace
•  perf c2c (new!)
•  perf orace (new!)
•  …

Links & References
•  perf_events
•  Kernel source: tools/perf/DocumentaCon
•  hUps://perf.wiki.kernel.org/index.php/Main_Page
•  hUp://www.brendangregg.com/perf.html
•  hUp://web.eece.maine.edu/~vweaver/projects/perf_events/
•  Mailing list hUp://vger.kernel.org/vger-lists.html#linux-perf-users
•  perf-tools: hUps://github.com/brendangregg/perf-tools
•  PMU tools: hUps://github.com/andikleen/pmu-tools
•  perf, orace, and more: hUp://www.brendangregg.com/linuxperf.html
•  Java frame pointer patch
•  hUp://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2014-December/016477.html
•  hUps://bugs.openjdk.java.net/browse/JDK-8068945
•  Node.js: hUp://techblog.ne9lix.com/2014/11/nodejs-in-ﬂames.html
•  Methodology: hUp://www.brendangregg.com/methodology.html
•  Flame graphs: hUp://www.brendangregg.com/ﬂamegraphs.html
•  Heat maps: hUp://www.brendangregg.com/heatmaps.html
•  eBPF: hUp://lwn.net/Ar/cles/603983/

Thank You
–  Ques/ons?
–  hUp://www.brendangregg.com
–  hUp://slideshare.net/brendangregg
–  bgregg@ne9lix.com
–  @brendangregg

Kernel Recipes 2017: Using Linux perf at Netflix

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Kernel Recipes 2017: Using Linux perf at Netflix