Re: Flush stdin

From	Marko Rauhamaa <marko@pacujo.net>
Newsgroups	comp.lang.python
Subject	Re: Flush stdin
Date	2014-10-21 07:41 +0300
Organization	A noiseless patient Spider
Message-ID	<87h9yyf318.fsf@elektro.pacujo.net> (permalink)
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Dan Stromberg <drsalists@gmail.com>:

> Often with TCP protocols, line buffered is preferred to character
> buffered,

Terminal devices support line buffering on write.

Line buffering on read is an illusion created by higher-level libraries.
The low-level read function reads in blocks of bytes.

> Also, it's a straightforward way of framing your data, to avoid
> getting messed up by Nagle or fragmentation.

Nagle affects the communication between the peer OS kernels and isn't
directly related to anything the application does. Also, Nagle doesn't
play any role with pipes.

>> ========================================================================
>> $ bash ./test.sh | strace python3 ./test.py
>> ...
>> read(0, "x", 4096)                      = 1
>> read(0, "x", 4096)                      = 1
>> read(0, "x", 4096)                      = 1
>> read(0, "x", 4096)                      = 1
>> read(0, "x", 4096)                      = 1
>> fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 3), ...}) = 0
>> mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
>> 0) = 0x7f3143bab000
>> write(1, "120\n", 4120
>> )                    = 4
>> ...
> ========================================================================
>
> This is tremendously inefficient.  It demands a context switch for
> every character.

Inefficiency isn't an issue when you generate one byte a second. If data
were generated at a brisker pace, "read(0, ..., 4096)" could get more
bytes at a time. Notice that even if the Python code requests 5 bytes,
CPython requests up to 4096 bytes in a single read.


Marko

Thread

Re: Flush stdin Cameron Simpson <cs@zip.com.au> - 2014-10-20 14:56 +1100
  Re: Flush stdin Marko Rauhamaa <marko@pacujo.net> - 2014-10-20 07:45 +0300
    Re: Flush stdin Dan Stromberg <drsalists@gmail.com> - 2014-10-20 14:59 -0700
      Re: Flush stdin Marko Rauhamaa <marko@pacujo.net> - 2014-10-21 01:54 +0300
    Re: Flush stdin Dan Stromberg <drsalists@gmail.com> - 2014-10-20 15:52 -0700
      Re: Flush stdin Marko Rauhamaa <marko@pacujo.net> - 2014-10-21 02:18 +0300
        Re: Flush stdin Dan Stromberg <drsalists@gmail.com> - 2014-10-20 17:30 -0700
          Re: Flush stdin Marko Rauhamaa <marko@pacujo.net> - 2014-10-21 07:41 +0300
            Re: Flush stdin Dan Stromberg <drsalists@gmail.com> - 2014-10-21 16:16 -0700
              Re: Flush stdin Marko Rauhamaa <marko@pacujo.net> - 2014-10-22 08:38 +0300
                Re: Flush stdin Chris Angelico <rosuav@gmail.com> - 2014-10-22 17:23 +1100
              Re: Flush stdin Marko Rauhamaa <marko@pacujo.net> - 2014-10-22 13:57 +0300
            Re: Flush stdin random832@fastmail.us - 2014-10-22 11:01 -0400
              Re: Flush stdin Marko Rauhamaa <marko@pacujo.net> - 2014-10-22 18:28 +0300

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