performance regression in socket getsockaddrarg()

[neologix]

BPO	22127
Nosy	@gvanrossum, @loewis, @ncoghlan, @pitrou, @vstinner, @serhiy-storchaka
Files	skip_idna.diff skip_idna_alt.patch skip_idna.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-08-05.14:23:02.273>
created_at = <Date 2014-08-03.08:17:29.004>
labels = ['library', 'performance']
title = 'performance regression in socket getsockaddrarg()'
updated_at = <Date 2014-08-05.14:23:02.272>
user = 'https://bugs.python.org/neologix'

bugs.python.org fields:

activity = <Date 2014-08-05.14:23:02.272>
actor = 'loewis'
assignee = 'none'
closed = True
closed_date = <Date 2014-08-05.14:23:02.273>
closer = 'loewis'
components = ['Library (Lib)']
creation = <Date 2014-08-03.08:17:29.004>
creator = 'neologix'
dependencies = []
files = ['36253', '36254', '36267']
hgrepos = []
issue_num = 22127
keywords = ['patch']
message_count = 24.0
messages = ['224618', '224634', '224658', '224665', '224666', '224674', '224694', '224696', '224709', '224715', '224717', '224721', '224730', '224735', '224737', '224739', '224740', '224818', '224819', '224824', '224828', '224830', '224831', '224832']
nosy_count = 8.0
nosy_names = ['gvanrossum', 'loewis', 'ncoghlan', 'pitrou', 'vstinner', 'neologix', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue22127'
versions = ['Python 3.4', 'Python 3.5']

[neologix]

I noticed that socket.sendto() got noticably slower since 3.4 (at least), compared to 2.7:

2.7:
$ ./python -m timeit -s "import socket; s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM); DATA = b'hello'; TARGET=('127.0.0.1', 4242)" "s.sendto(DATA, TARGET)"
100000 loops, best of 3: 15.8 usec per loop

3.4:
$ ./python -m timeit -s "import socket; s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM); DATA = b'hello'; TARGET=('127.0.0.1', 4242)" "s.sendto(DATA, TARGET)"
10000 loops, best of 3: 25.9 usec per loop

A profile reveals this:
2.7:
100065 function calls in 2.268 seconds

Ordered by: cumulative time

ncalls tottime percall cumtime percall filename:lineno(function)
1 0.361 0.361 2.268 2.268 test_send.py:1(<module>)
100000 1.895 0.000 1.895 0.000 {method 'sendto' of '_socket.socket' objects}

3.4:
906015 function calls (905975 primitive calls) in 6.132 seconds

Ordered by: cumulative time

ncalls tottime percall cumtime percall filename:lineno(function)
5/1 0.000 0.000 6.132 6.132 {built-in method exec}
1 0.334 0.334 6.132 6.132 test_send.py:1(<module>)
100000 2.347 0.000 5.769 0.000 {method 'sendto' of '_socket.socket' objects}
100000 1.991 0.000 3.411 0.000 idna.py:147(encode)
500086 0.894 0.000 0.894 0.000 {built-in method len}
100000 0.269 0.000 0.269 0.000 {method 'encode' of 'str' objects}
100000 0.257 0.000 0.257 0.000 {method 'split' of 'bytes' objects}

As can be seen, it's the IDNA encoding which takes a long time, and doesn't appear in the 2.7 profile.
The parsing code (including idna codec) is present in both versions though:
"""
static int
getsockaddrarg(PySocketSockObject *s, PyObject *args,
               struct sockaddr *addr_ret, int *len_ret)
[...]
        if (!PyArg_ParseTuple(args, "eti:getsockaddrarg",
                              "idna", &host, &port))
            return 0;
"""

I'm currently bisecting the commit, but I'm not familiar with encoding stuff.
Is it expected that the IDNA encoding be called when passed an ascii string?

[pitrou]

IDNA encoding is quite slow (see 6e1071ed4c66). I'm surprised we accept general hosnames in sendto(), though (rather than plain IP addresses). 25 µs per call is a lot for such a function.

[vstinner]

For Python, the encoder is only used when you pass a Unicode string.

[neologix]

For Python, the encoder is only used when you pass a Unicode string.

Hm...
I'm passing ('127.0.0.1', 4242)as destination, and you can see in the
above profile that the idna encode function is called.
This doesn't occur with 2.7.

[neologix]

OK, I think I see what you mean:

$ ./python -m timeit -s "import socket; s =
socket.socket(socket.AF_INET, socket.SOCK_DGRAM)" "s.sendto(b'hello',
('127.0.0.1', 4242))"10000 loops, best of 3: 44.7 usec per loop
$ ./python -m timeit -s "import socket; s =
socket.socket(socket.AF_INET, socket.SOCK_DGRAM)" "s.sendto(b'hello',
(b'127.0.0.1', 4242))"
10000 loops, best of 3: 23.7 usec per loop

That's really surprising, especially since gethostbyname() and
getaddrinfo() seem to return strings:
$ ./python -m timeit -s "import socket; s =
socket.socket(socket.AF_INET, socket.SOCK_DGRAM);
addr=socket.gethostbyname('127.0.0.1')" "s.sendto(b'hello', (addr,
4242))"

$ ./python -c "import socket; print(type(socket.gethostbyname('127.0.0.1')))"
<class 'str'>

[pitrou]

Note that even the bytes version is still quite slow. UDP is used for light-weight protocols where you may send thousands or more messages per second. I'd be curious what the sendto() performance is in raw C.

[vstinner]

"Abc" is a bytes string in Python 2 and an Unicode string in Python 3.

[neologix]

Note that even the bytes version is still quite slow. UDP is used for light-weight protocols where you may send thousands or more messages per second. I'd be curious what the sendto() performance is in raw C.

Ah, I wouldn't rely on the absolyte values, my computer is *slow*.

On a more recent machine, I get this:
100000 loops, best of 3: 8.82 usec per loop

Whereas a C loop gives a 4usec per loop.

"Abc" is a bytes string in Python 2 and an Unicode string in Python 3.

Sure, but why do getaddrinfo() and gethostbyname() return strings then?

This means that someone using:

addr = getaddrinfo(...)
sendto(DATA, addr)

Will pay the idna encoding upon every call to sendto().

[serhiy-storchaka]

Perhaps it is time to add support of ipaddress objects in socket functions. Then we could avoid address parsing in tight loop not only for Unicode strings, but for bytes strings too.

s = socket.socket(...)
addr = ipaddress.ip_address(ipaddress.getaddrinfo(...))
for ...:
    s.sendto(DATA, (addr, port))

[pitrou]

Perhaps it is time to add support of ipaddress objects in socket functions.

What I was thinking too :-)
However, beware the parsing cost of ipaddress objects themselves.

One common pattern when doing UDP networking is the following:

  def datagram_received(self, remote_addr, data):
      # process data
      ...
      self.send_to(remote_addr, response_data)

If you want to pass an ipaddress object to send_to, you have to make it so that datagram_received() gives you an ipaddress object too.

Perhaps we need a more low-level solution, e.g. a parsing cache integrated in the C socket module.

[neologix]

Parsing a bytes object i.e. b'127.0.0.1' is done by inet_pton(), so
it's probably cheap (compared to a syscall).

If we had getaddrinfo() and gethostbyname() return bytes instead of
strings, it would be a huge gain.

[loewis]

Charles-François: you get the idna overhead in 2.7, too, by specifying u'127.0.0.1' as the address.

The idna overhead could be bypassed fairly easily in C by:

1. checking that the string is an ASCII string (this is possible in constant time, in 3.x)
2. directly passing the ASCII string to setipaddr (leaving any error detection to this routine)

Before adding caching, I'd check whether a cache lookup is actually faster than calling inet_pton.