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he 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.
[There is a short thread discussing this issue on python-dev; Subject: "http.client Nagle/delayed-ack optimization"; Date: Dec 15, 2012]

Thanks for the patch. Perhaps our MSS value should be an upper bound of common values? Apparently for a localhost connection TCP_MAXSEG gives 16384 here (but I don't know if the http.client optimization is important for localhost connections).
Also, it would be nice to add an unit test in Lib/test/test_httplib.py.

> Thanks for the patch. Perhaps our MSS value should be an upper bound of common values? Apparently for a localhost connection TCP_MAXSEG gives 16384 here (but I don't know if the http.client optimization is important for localhost connections).
According to http://en.wikipedia.org/wiki/Maximum_transmission_unit,
the MTU (from which the MSS is derived) for a wireless NIC is around
8000, and you can have jumbo frames on Ethernet up to 9000, so a value
of 2K might be too small (since the goal is to avoid sending a payload
less than MSS with its own send() syscall).
So I think a value like 16K should be OK (the downside to using a
large value is the extra copying, but OTOH it will incur less copying
than now).
Note that the ideal fix for this would be scatter-gather I/O with
sendmsg(): both the header and the payload could be sent in the same
segment, without any extra copying. But I'm not sure it's worth it
here.
A note on the comment:
"""
722 # TCP Maximum Segment Size (MSS) is a TCP stack parameter. There
723 # is no simple and efficient platform independent mechanism for
724 # determining the MSS, so instead a reasonable estimate is chosen.
"""
The MSS is not really a TCP stack parameter (i.e. it's not fixed by
the host like socket buffers, etc): it's usually negociated between
the hosts (and with path MTU discovery or IPv6 it depends on the path
MTU). That's why it's hard to guess ahead of time.

I've updated the patch based on Charles-François and Antoine's feedback.
Primarily this increase the estimated MSS value to 16KiB.
Additionally tests are added and comments improved.
Thanks for the feedback.

New changeset 002bf9857278 by Antoine Pitrou in branch 'default':
Issue #16833: In http.client.HTTPConnection, do not concatenate the request headers and body when the payload exceeds 16 KB, since it can consume more memory for no benefit.
http://hg.python.org/cpython/rev/002bf9857278 

Patch looks good to me, I've committed it. Thank you!

By the way, could you sign a contributor agreement?
http://www.python.org/psf/contrib/
Thank you.

Jesus, why are you reopening this issue?

Sorry, firefox is playing with me. As usual :). Just writing about i in python-dev.
Thanks for the heads up.