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[Python-Dev] http.client Nagle/delayed-ack optimization

Antoine Pitrou solipsis at pitrou.net
Fri Dec 14 20:27:00 CET 2012 
On Sat, 15 Dec 2012 06:17:19 +1100
Ben Leslie <benno at benno.id.au> wrote:
> The http.client HTTPConnection._send_output method has an optimization for
> avoiding bad interactions between delayed-ack and the Nagle algorithm:
> 
> http://hg.python.org/cpython/file/f32f67d26035/Lib/http/client.py#l884
> 
> Unfortunately this interacts rather poorly if the case where the
> message_body is a bytes instance and is rather large.
> 
> If the message_body is bytes it is appended to the headers, which causes a
> copy of the data. When message_body is large this duplication of data can
> cause a significant spike in memory usage.
> 
> (In my particular case I was uploading a 200MB file to 30 hosts at the same
> leading to memory spikes over 6GB.
> 
> I've solved this by subclassing and removing the optimization, however I'd
> appreciate thoughts on how this could best be solved in the library itself.
> 
> Options I have thought of are:
> 
> 1: Have some size threshold on the copy. A little bit too much magic.
> Unclear what the size threshold should be.

I think a hardcoded threshold is the right thing to do. It doesn't
sound very useful to try doing a single send() call when you have a
large chunk of data (say, more than 1 MB).

Regards

Antoine.
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