Slow performance when sending a large file upload request via proxy_pass

[pcd]

I'm trying to diagnose some strange behavior in my web app, and at the
moment it seems like nginx may be at fault, though I'd be happy to learn
otherwise.

On the client side, I'm using flow.js (https://github.com/flowjs/flow.js) to
upload a file to the server. This library should allow me to upload very
large files by splitting them up into (by default) 1MB chunks, and sending
each chunk as a standard file form upload request.

On the server, I am connecting to a Python WSGI server (gunicorn) via
try_files / proxy_pass. The configuration is very standard:

    location / {
        root   /var/www;
        index  index.html index.htm;
        try_files $uri $uri/ @proxy_to_app;
    }

    location @proxy_to_app {
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header Host $http_host;
            proxy_redirect off;

            proxy_pass   http://app_server;
    }

The Python code is pretty simple, mainly just opening the file and writing
the data. According to the gunicorn access log, each request takes around
135ms:

    127.0.0.1 - - [17/Jul/2016:05:07:07 +0000] "POST /files HTTP/1.0" 200 -
... 0.135206
    127.0.0.1 - - [17/Jul/2016:05:07:07 +0000] "POST /files HTTP/1.0" 200 -
... 0.136749
    127.0.0.1 - - [17/Jul/2016:05:07:07 +0000] "POST /files HTTP/1.0" 200 -
... 0.137314

But in the nginx access log, the $request_time varies wildly and is usually
very large:

    10.0.0.0 - - [17/Jul/2016:05:07:06 +0000] "POST /files HTTP/1.1" 200 ...
0.956
    10.0.0.0 - - [17/Jul/2016:05:07:07 +0000] "POST /files HTTP/1.1" 200 ...
0.553
    10.0.0.0 - - [17/Jul/2016:05:07:07 +0000] "POST /files HTTP/1.1" 200 …
0.888

At first I thought it might be the network itself taking a long time to send
the data, but looking at the network logs in the browser doesn’t seem to
bear this out. Once the socket connection is established, Chrome says that
the request time is often as low as 8ms, with the extra ~.5s-1s spent
waiting for a response.

So the question is, what is nginx doing during all that extra time? On
normal (small) requests, the times in the two logs are identical, but even
dialing down the chunk size to flow.js to 128kb or 64kb results in a delay
in nginx, and it's making it take way too long to upload these files (I
can't just set the chunk size to something super small like 4kb, because the
overhead of making so many requests makes the uploads slower).

I've tried messing with various configuration options including
proxy_buffer_size and proxy_request_buffering, to no effect.

Any ideas on next steps for how I could begin to diagnose this?

Extra info:

    CentOS 7, running on AWS

    nginx version: nginx/1.10.1
    built by gcc 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC)
    built with OpenSSL 1.0.1e-fips 11 Feb 2013
    TLS SNI support enabled
    configure arguments: --prefix=/etc/nginx --sbin-path=/usr/sbin/nginx
--modules-path=/usr/lib64/nginx/modules --conf-path=/etc/nginx/nginx.conf
--error-log-path=/var/log/nginx/error.log
--http-log-path=/var/log/nginx/access.log --pid-path=/var/run/nginx.pid
--lock-path=/var/run/nginx.lock
--http-client-body-temp-path=/var/cache/nginx/client_temp
--http-proxy-temp-path=/var/cache/nginx/proxy_temp
--http-fastcgi-temp-path=/var/cache/nginx/fastcgi_temp
--http-uwsgi-temp-path=/var/cache/nginx/uwsgi_temp
--http-scgi-temp-path=/var/cache/nginx/scgi_temp --user=nginx --group=nginx
--with-http_ssl_module --with-http_realip_module --with-http_addition_module
--with-http_sub_module --with-http_dav_module --with-http_flv_module
--with-http_mp4_module --with-http_gunzip_module
--with-http_gzip_static_module --with-http_random_index_module
--with-http_secure_link_module --with-http_stub_status_module
--with-http_auth_request_module --with-http_xslt_module=dynamic
--with-http_image_filter_module=dynamic --with-http_geoip_module=dynamic
--with-http_perl_module=dynamic --add-dynamic-module=njs-1c50334fbea6/nginx
--with-threads --with-stream --with-stream_ssl_module
--with-http_slice_module --with-mail --with-mail_ssl_module --with-file-aio
--with-ipv6 --with-http_v2_module --with-cc-opt='-O2 -g -pipe -Wall
-Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong
--param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic'

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,268317,268317#msg-268317