https://github.com/pyconsk/2016-slides
PyCon SK 2016 - March 2016 1
DTrace and Python
Jesús Cea Avión
jcea@jcea.es
@jcea
httpS://www.jcea.es/
httpS://blog.jcea.es/
PyCon SK 2016 - March 2016 2
Jesús Cea Avión
● Programming in Python since 1996 (Python 1.4).
● Core Developer since 2008 (Python 2.6 and 3.0).
● Founder of Python Madrid, Python Vigo and
Python España association.
● Solaris user since 1990, SysOP since 1996.
● Consultant and freelance always searching for new
and interesting challenges. Hire me! :-)
PyCon SK 2016 - March 2016 3
Overview of this talk
● What is DTrace?
● Quick & dirty overview.
● Relevance for Python.
● Probes in the interpreter.
● Examples tracing a Python program.
● Examples tracing the entire stack, including OS.
● Future. Help!
● More probes.
● Porting to other DTrace supported platforms.
PyCon SK 2016 - March 2016 4
Python
● You already know about this…
DTrace
● Comprehensive full system dynamic tracing
framework developed by Sun Microsystems for
Solaris.
● Virtually zero performance impact when not in use.
● Safe to use in production.
● Available on Solaris and derivatives, FreeBSD,
NetBSD, Mac OS X, Oracle Linux.
PyCon SK 2016 - March 2016 5
DTrace
● Operating system, libraries and programs can
define “probes”:
# dtrace l | wc l
259438
● Can fire at machine language function call/return.
● Can fire at arbitrary machine language instruction.
● SAFE to use in production.
● (Almost) zero performance impact when not in use.
PyCon SK 2016 - March 2016 6
DTrace
● DTrace language is safe, read-only. (*)
● Probes everywhere:
● Syscall, virtual memory, CPU scheduler, network,
locks, disk...
● High level probes.
● Dedicated providers. For instance, Python.
● Dynamic providers. For instance, sampling profile.
● Synthetic providers: process defined probes.
PyCon SK 2016 - March 2016 7
DTrace
● Simple language to activate arbitrary probes and
execute code when the event “fires”.
● Speculative tracing.
● It doesn’t require process collaboration, but helpful.
● Native aggregation functions.
● Associative arrays.
● Excellent documentation.
● DevOps paradise.
PyCon SK 2016 - March 2016 8
DTrace examples
Show me the processes doing “fsync()” calls:
# dtrace l P syscall | wc l
471 ← Include entry/return + heading
# dtrace n 'syscall::fdsync:entry {printf("%s",
execname);}'
dtrace: description 'syscall::fdsync:entry ' matched
1 probe
CPU ID FUNCTION:NAME
0 58858 fdsync:entry lmtp
[...]
4 58858 fdsync:entry lmtp
7 58858 fdsync:entry cleanup
PyCon SK 2016 - March 2016 9
DTrace examples
Show me “fsync()” duration stats:
# dtrace n 'syscall::fdsync:entry {self>t =
timestamp;} syscall::fdsync:return {@t =
quantize(timestampself>t);}'
dtrace: description 'syscall::fdsync:entry ' matched
2 probes
^C
value Distribution count
262144 | 0
524288 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 7
1048576 | 0
2097152 | 0
4194304 | 0
8388608 | 0
16777216 |@@@@@ 1
33554432 | 0
PyCon SK 2016 - March 2016 10
DTrace examples
Peek inside a process:
# dtrace l n pid25590:::entry | wc l
21204
# dtrace l n pid25590:libssl.so.1.0.0::entry | wc l
649
# dtrace n
pid9498:libssl.so.1.0.0:ssl_verify_cert_chain:entry
dtrace: description
'pid9498:libssl.so.1.0.0:ssl_verify_cert_chain:entry
' matched 1 probe
PyCon SK 2016 - March 2016 11
DTrace examples
● Sampling profiler:
# dtrace n 'profile997 /pid == 9912/ {jstack();}'
● Show me CPU use of a particular process:
# dtrace n 'BEGIN {oncpu=0;totalcpu=0;} profile
997 /pid == 10354/ {oncpu+=1;} profile997
{totalcpu+=1;} END {printf("%d %d", totalcpu,
oncpu);}'
dtrace: description 'BEGIN ' matched 4 probes
^C
CPU ID FUNCTION:NAME
3 2 :END 78972 9871
PyCon SK 2016 - March 2016 12
DTrace probes in Python
● Instrumented interpreter for better information:
# dtrace l P python9134
ID PROVIDER MODULE FUNCTION NAME
59421 python9134 libpython3.5m.so.1.0 PyEval_EvalFrameEx functionentry
59422 python9134 libpython3.5m.so.1.0 PyEval_EvalFrameEx functionreturn
59423 python9134 libpython3.5m.so.1.0 _PyGC_CollectNoFail gcdone
59424 python9134 libpython3.5m.so.1.0 PyGC_Collect gcdone
59425 python9134 libpython3.5m.so.1.0 gc_collect gcdone
59426 python9134 libpython3.5m.so.1.0 collect_with_callback gcdone
59427 python9134 libpython3.5m.so.1.0 _PyGC_CollectNoFail gcstart
59428 python9134 libpython3.5m.so.1.0 PyGC_Collect gcstart
59429 python9134 libpython3.5m.so.1.0 gc_collect gcstart
59430 python9134 libpython3.5m.so.1.0 collect_with_callback gcstart
59431 python9134 libpython3.5m.so.1.0 subtype_dealloc instancedeletedone
59432 python9134 libpython3.5m.so.1.0 subtype_dealloc instancedeletestart
59433 python9134 libpython3.5m.so.1.0 PyType_GenericAlloc instancenewdone
59434 python9134 libpython3.5m.so.1.0 PyType_GenericAlloc instancenewstart
59435 python9134 libpython3.5m.so.1.0 PyEval_EvalFrameEx line
PyCon SK 2016 - March 2016 13
DTrace probes in Python
● Current probes:
● line
● function-entry
● function-return
● gc-start
● gc-done
● instance-new-start
● instance-new-done
● instance-delete-start
● instance-delete-done
PyCon SK 2016 - March 2016 14
Examples of DTrace in Python
● Tell me where a particular library call is done:
# dtrace n 'python12042:::functionentry
/copyinstr(arg0)=="/usr/local/lib/python3.5/ssl.py
" && copyinstr(a1)=="getpeercert"/ {jstack(100,
10000);}' | grep '['
dtrace: description 'python12042:::functionentry '
matched 1 probe
[ python3.5/ssl.py:805 (getpeercert) ]
[ urllib3/connection.py:259 (connect) ]
[…]
[ requests/adapters.py:376 (send) ]
[…]
[ requests/api.py:53 (request) ]
[…]
PyCon SK 2016 - March 2016 15
Examples of DTrace in Python
● Tell me how long are garbage collections:
# dtrace n 'python12042:::gcstart {self>t =
timestamp;} python12042:::gcdone {printf("%uus",
timestampself>t);}'
dtrace: description 'python12042:::gcstart '
matched 8 probes
CPU ID FUNCTION:NAME
5 101930 collect_with_callback:gcdone 8480us
5 101930 collect_with_callback:gcdone 3062us
5 101930 collect_with_callback:gcdone 1891us
● What Python function fires most GC?
● How frequent are GC?
PyCon SK 2016 - March 2016 16
Examples of DTrace in Python
● Poor man memory “Leak” detector:
# dtrace n 'python12042:::instancenewstart
{@[copyinstr(arg0)] = sum(1);}
python12042:::instancedeletedone
{@[copyinstr(arg0)] = sum(1);}'
dtrace: description 'python12042:::instancenewstart ' matched 2 probes
^C
[…]
_GeneratorContextManager 0
socket 0
BufferedSubFile 2
FeedParser 2
HTTPMessage 2
PyCon SK 2016 - March 2016 17
Examples of DTrace in Python
● Trace a Apache MOD_WSGI process:
# dtrace n 'python25589:::functionentry
/copyinstr(arg1)=="application"/ {self>f=1;}
python25589:::functionreturn
/copyinstr(arg1)=="application"/ {self>f=0;}
python25589:::functionentry /self>f/ {printf("%s",
copyinstr(arg1));}'
dtrace: matched 3 probes
CPU ID FUNCTION:NAME
3 4350 PyEval_EvalFrameEx:functionentry application
3 4350 PyEval_EvalFrameEx:functionentry salida
● What operating system calls are being slow?
● Where are we being preempt by the OS? For how
long? Why?
PyCon SK 2016 - March 2016 18
Examples DTrace: Python + OS
● Show me where I am being blocked
(synchronization object):
# dtrace n 'sched:::sleep /pid==14857/
{printf("[BLOCKED] %d ", tid); jstack();}' | grep
"["
dtrace: matched 7 probes
2 5025 cv_block:sleep [START] 2
[ python3.5/threading.py:293 (wait) ]
[ python3.5/queue.py:164 (get) ]
[ python3.5/concurrent/futures/thread.py:64 (_worker) ]
[...]
● CPU accounting per Python Thread.
● What processes are stealing my CPU?
● Examine lock contention, even GIL.
PyCon SK 2016 - March 2016 19
Examples DTrace: Python + OS
● What code is actually accessing the disk, not
getting data from cache?
# dtrace n 'io:::start /pid==14857/ {jstack();}'
dtrace: description 'io:::start ' matched 6 probes
CPU ID FUNCTION:NAME
6 5049 bdev_strategy:start
libc.so.1`_read+0x15
libpython3.5m.so.1.0`_Py_read+0x4b
libpython3.5m.so.1.0`_io_FileIO_readall_impl.isra.8+0xeb
libpython3.5m.so.1.0`PyCFunction_Call+0xca
libpython3.5m.so.1.0`PyObject_Call+0x68
libpython3.5m.so.1.0`PyObject_CallMethodObjArgs+0xa2
libpython3.5m.so.1.0`_io__Buffered_read+0x47f
libpython3.5m.so.1.0`PyCFunction_Call+0xd9
libpython3.5m.so.1.0`PyEval_EvalFrameEx+0xa051
[ <stdin>:1 (<module>) ] ← open(“file”, “rb”).read()
libpython3.5m.so.1.0`_PyEval_EvalCodeWithName+0xb31
libpython3.5m.so.1.0`PyEval_EvalCode+0x30
libpython3.5m.so.1.0`PyRun_InteractiveOneObject+0x1a5
libpython3.5m.so.1.0`PyRun_InteractiveLoopFlags+0x7d
libpython3.5m.so.1.0`PyRun_AnyFileExFlags+0x40
libpython3.5m.so.1.0`Py_Main+0xe21
python3.5`main+0x170
python3.5`_start+0x80
PyCon SK 2016 - March 2016 20
Notice:
● You don’t modify the source code. You don’t even
need source code access. No collaboration.
● If you have OS source code, you are GOD!.
● You enable the tracing surgically, when you need it
and for the time you need it, from a separate
terminal.
● The process continues unaltered, in production.
● Exploratory tracing: hypothesis and fast validation.
● Full system visibility.
PyCon SK 2016 - March 2016 21
More: Python USDT
(Userland Statically Defined Tracing)
● Your python code can define high level probes:
● client connect, request start, job enqueued,
download completed, ...
● Activate logging surgically, on demand, with the
daemon running undisturbed.
● You can create individual entry/return probes per
function/method with “@fbt” decorator.
● BAD: Stale? code, no documentation. Partial 3.x.
PyCon SK 2016 - March 2016 22
More: Python USDT
(Userland Statically Defined Tracing)
Python 2.7.11 (dtraceissue13405_2.7:8c5948409bbe,
Mar 3 2016, 04:49:13)
[GCC 5.3.0] on sunos5
Type "help", "copyright", "credits" or "license" for
more information.
>>> import os
>>> from usdt.tracer import fbt
>>> @fbt
... def example(v) :
... pass
...
>>> os.getpid()
24793
>>> example("hello world!")
PyCon SK 2016 - March 2016 23
More: Python USDT
(Userland Statically Defined Tracing)
# dtrace l P pythonfbt24793
ID PROVIDER MODULE FUNCTION NAME
59327 pythonfbt24793 fbt example entry
59328 pythonfbt24793 fbt example return
# dtrace n 'pythonfbt24793::example:* {}'
dtrace: description 'pythonfbt24793::example:* '
matched 2 probes
CPU ID FUNCTION:NAME
5 59327 example:entry
5 59328 example:return
PyCon SK 2016 - March 2016 24
Future:
● Support all DTrace platforms. Sprint tomorrow!
● Add more Python probes in the interpreter and C
módules:
● GIL, Threading module, import machinery...
● Python programs should be able to create
personalized dynamic probes. DONE: PythonUSDT.
● Challenge: integrate with mainstream CPython.
PyCon SK 2016 - March 2016 25
Performance
● When not enabled, performance hit is VERY low:
DISABLED ENABLED
0xfee9f79a <+2954>: jne 0xfee9ede3
0xfee9f7a0 <+2960>: xor %eax,%eax
0xfee9f7a2 <+2962>: nop int3
0xfee9f7a3 <+2963>: nop
0xfee9f7a4 <+2964>: nop
0xfee9f7a5 <+2965>: test %eax,%eax
0xfee9f7a7 <+2967>: mov 0x60(%ebp),%edx
0xfee9f7aa <+2970>: jne 0xfeea840e
● Current Python USDT implemented in Python,
performance hit even when probes are not
enabled. Python 2.7, function call+return: x143.
PyCon SK 2016 - March 2016 26
DTrace Sprint
tomorrow
March, 13th
Main target:
Correctly support FreeBSD,
NetBSD and Mac OS X.
PyCon SK 2016 - March 2016 27
Additional References
● Python documentation and code:
https://www.jcea.es/artic/python_dtrace.htm
● General documentation:
https://en.wikipedia.org/wiki/DTrace
http://dtrace.org/guide/preface.html
http://dtrace.org/blogs/
https://wiki.freebsd.org/DTrace/One-Liners
http://dtracebook.com/index.php/Main_Page
● Python USDT:
https://pypi.python.org/pypi/usdt/
https://github.com/nshalman/python-usdt/
https://github.com/chrisa/libusdt
● Linux:
https://github.com/dtrace4linux/linux
https://docs.oracle.com/cd/E37670_01/E37355/html/ol_dtrace.html
PyCon SK 2016 - March 2016 28
Questions?
● What is “Speculative Tracing”?
● Stack traces and Mac OS X.
https://www.mail-archive.com/dtrace-discuss@opensolaris.org/msg04668.html
● What is needed to integrate with mainline
CPython?.
● Other interpreters?
● SystemTap synergies.
● DTrace in Linux?
PyCon SK 2016 - March 2016 29
¡Thank you!
Jesús Cea Avión
jcea@jcea.es
@jcea
httpS://www.jcea.es/
httpS://blog.jcea.es/