[issue38699] socket: change listen() default backlog from 128 to 4096?

[STINNER Victor]

STINNER Victor  added the comment:

There is no clear consensus to change socket.listen() default backlog, so I 
close the issue.

> Anyway asyncio at least should probably update its default.

If someone cares, please open a separated issue ;-)

> To be clear: I do still think that using a large value by default, and 
> clamping user input values at 65535, would be improvements. I just don't 
> think they're important enough to spend energy arguing about it :-).

I dislike having to workaround operating system bugs. If the function 
misbehaves, I would prefer to see the OS being fixed, than Python trying to 
guess which value is supposed to work or not.

   int listen(int sockfd, int backlog);

backlog type is an int: its maximum value is INT_MAX. If the kernel rejects 
values larger than 65535, the kernel and/or the libc should reject such value.

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[issue38699] socket: change listen() default backlog from 128 to 4096?

[Nathaniel Smith]

Nathaniel Smith  added the comment:

Yeah, I don't think it matters that much. There are lots of random gotchas that 
you have to watch out for using the socket module.

To be clear: I do still think that using a large value by default, and clamping 
user input values at 65535, would be improvements. I just don't think they're 
important enough to spend energy arguing about it :-).

The reason I'm not worried about large backlogs wasting kernel memory is that 
servers all call accept as fast as they can. This means any incoming connection 
flood will end up taking memory regardless of the size of the kernel-side 
buffer. And I'm pretty sure the kernel buffer ia a linked list, so setting it 
to a large value doesn't cost anything if you're not using it.

Anyway asyncio at least should probably update its default.

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[issue38699] socket: change listen() default backlog from 128 to 4096?

[STINNER Victor]

STINNER Victor  added the comment:

gunicorn, tornado, uwsgi, trio are specialized use cases which expect high 
concurrency.

Maybe a tradeoff may be to document the default and suggest different values 
depending on the use cases, and/or to point to documentation.

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[issue38699] socket: change listen() default backlog from 128 to 4096?

[STINNER Victor]

STINNER Victor  added the comment:

> https://github.com/python-trio/trio/blob/master/trio/_highlevel_open_tcp_listeners.py

Extract:

# A large backlog can also use a bit more kernel memory, but this seems fairly
# negligible these days.

That's the main question here. Python is used on various platforms for various 
use cases.

Extract of the current C code:

/* We try to choose a default backlog high enough to avoid connection drops
 * for common workloads, yet not too high to limit resource usage. */
int backlog = Py_MIN(SOMAXCONN, 128);

Maybe the status quo is just fine and this issue should be closed. It's trivial 
to override the default. This issue is stricted to the *default* value.

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[issue38699] socket: change listen() default backlog from 128 to 4096?

[Nathaniel Smith]

Nathaniel Smith  added the comment:

Another subtlety that that code handles, and that the stdlib socket module 
might also want to handle: if the user passes in a custom backlog argument 
that's >65535, then we silently replace it with 66535 before passing it to the 
system. The reason is that many systems will silently truncate this value to 16 
bits, so if a user explicitly passes in 65536, what they get is a backlog of 1, 
which is probably not what they were hoping for.

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[issue38699] socket: change listen() default backlog from 128 to 4096?

[Nathaniel Smith]

Nathaniel Smith  added the comment:

Trio uses 65535 as the default backlog argument. There's a big comment here 
explaining why: 
https://github.com/python-trio/trio/blob/master/trio/_highlevel_open_tcp_listeners.py

This doesn't actually set the backlog to 65635; instead it tells the kernel to 
use the biggest backlog that it feels comfortable with (e.g. 
/proc/sys/net/core/somaxconn on Linux).

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[issue38699] socket: change listen() default backlog from 128 to 4096?

[STINNER Victor]

Change by STINNER Victor :


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[issue38699] socket: change listen() default backlog from 128 to 4096?

[Ammar Askar]

Ammar Askar  added the comment:

Just for some more reference points from "production" python web servers:

* gunicorn - 2048 
(https://github.com/benoitc/gunicorn/blob/678b326dc030b450717ec505df69863dcd6fb716/docs/source/settings.rst#backlog)

* tornado - 128 
(https://github.com/tornadoweb/tornado/blob/c50aed0f96d92f9b0ef4cd0837c0104f140ca77e/tornado/tcpserver.py#L178)

* uwsgi - 100 
(https://github.com/unbit/uwsgi/blob/3149df02ed443131c54ea6afb29fcbb0ed4d1139/core/init.c#L115)

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[issue38699] socket: change listen() default backlog from 128 to 4096?

[STINNER Victor]

STINNER Victor  added the comment:

> I just saw an article suggesting to use 4096 instead:

In fact, it's a change in the Linux kernel directly proposed by Eric Dumazet 
(Google) for Linux kernel 5.5:
https://lore.kernel.org/netdev/20191030163620.140387-1-eduma...@google.com/

The change has already been merged upstream:

/* Maximum queue length specifiable by listen.  */
#define SOMAXCONN   4096

https://github.com/torvalds/linux/blob/a99d8080aaf358d5d23581244e5da23b35e340b9/include/linux/socket.h#L265

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[issue38699] socket: change listen() default backlog from 128 to 4096?

[STINNER Victor]

New submission from STINNER Victor :

Currently, socket.socket.listen() called with no argument limits the default 
backlog to 128:

/* We try to choose a default backlog high enough to avoid connection drops
 * for common workloads, yet not too high to limit resource usage. */
int backlog = Py_MIN(SOMAXCONN, 128);

I just saw an article suggesting to use 4096 instead:
https://blog.cloudflare.com/syn-packet-handling-in-the-wild/

On my Fedora 30, listen() manual page says:

   The  behavior of the backlog argument on TCP sockets changed with Linux
   2.2.  Now it specifies the  queue  length  for  completely  established
   sockets  waiting  to  be  accepted, instead of the number of incomplete
   connection requests.  The maximum length of the  queue  for  incomplete
   sockets  can be set using /proc/sys/net/ipv4/tcp_max_syn_backlog.  When
   syncookies are enabled there is no logical maximum length and this set‐
   ting is ignored.  See tcp(7) for more information.

   Ifthe   backlog   argument   is   greater   than   the   value   in
   /proc/sys/net/core/somaxconn, then it is  silently  truncated  to  that
   value;  the  default  value  in  this  file  is 128.  In kernels before
   2.4.25, this limit was a hard coded value, SOMAXCONN,  with  the  value
   128.

Python 3.8 new socket.create_server() calls sock.listen() by default: so use 
Py_MIN(SOMAXCONN, 128) by default.

--
components: Library (Lib)
messages: 356037
nosy: giampaolo.rodola, vstinner
priority: normal
severity: normal
status: open
title: socket: change listen() default backlog from 128 to 4096?
versions: Python 3.9

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