Nginx – is it normal for NGINX to accumulate connections in writing state

This is a graph showing connection count from nginx's stab_status, the drops in graph are server restarts done manually.

I find this behaviour strange, looks like some leakage. I think there are connections hanging that should be closed, but they stay in writing state.

Is this normal? Should I take an action? If so, in which direction?

I tried changing nginx's keepalive, and then some kernel settings like this https://gist.github.com/perusio/2154235

But this behaviour hasn't change.

The server is a VPS with one processor available and 1024mb ram.

EDIT1: versions

nginx version: nginx/1.6.2
built by gcc 4.8.2 (Ubuntu 4.8.2-19ubuntu1)
TLS SNI support enabled
configure arguments: –add-module=/root/ngx_pagespeed-release-1.9.32.3-beta –prefix=/etc/nginx –conf-path=/etc/nginx/nginx.conf –error-log-path=/var/log/nginx/error.log –http-client-body-temp-path=/var/lib/nginx/body –http-fastcgi-temp-path=/var/lib/nginx/fastcgi –http-log-path=/var/log/nginx/access.log –http-proxy-temp-path=/var/lib/nginx/proxy –http-scgi-temp-path=/var/lib/nginx/scgi –http-uwsgi-temp-path=/var/lib/nginx/uwsgi –lock-path=/var/lock/nginx.lock –pid-path=/var/run/nginx.pid –with-pcre-jit –with-http_ssl_module –without-mail_pop3_module –without-mail_smtp_module –without-mail_imap_module –without-http_scgi_module –with-ipv6 –with-http_stub_status_module –sbin-path=/usr/sbin/nginx –with-http_spdy_module

Note that it runs ngx_pagespeed

EDIT2: More info

Nginx is working as a reverse proxy for uwsgi/django app. I'll monitor uwsgi and post the results to see better if uwsgi has something to do.