I found: http://forum.nginx.org/read.php?2,220954,220989#msg-220989

    It represents global data and configuration of a event loop (that
    is, whole nginx). New cycle is created on a reconfiguration.

Testing with configuration reloading, this patch doesn't work:

    $ curl http://localhost:8050/mod_pagesped_example/styles/W.rewrite_css_images.css.pagespeed.cf.Hash.css
    .foo{background-image:url(http://localhost:8050/mod_pagespeed_example/images/xBikeCrashIcn.png.pagespeed.ic.2FihTuVNs_.png);width:100px;height:100px}
    $ /home/jefftk/nginx/sbin/nginx -s reload -c /home/jefftk/ngx_pagespeed/test/tmp/pagespeed_test.conf
    $ curl http://localhost:8050/mod_pagespeed_example/styles/W.rewrite_css_images.css.pagespeed.cf.Hash.css
2013/04/10 17:17:25 [alert] 31028#0: 1024 worker_connections are not enough
    curl: (56) Recv failure: Connection reset by peer
    $ telnet localhost 8050
    Trying 127.0.0.1...
    Connected to localhost.
    Escape character is '^]'.
    2013/04/10 17:18:56 [alert] 31563#0: 1024 worker_connections are not enough
    Connection closed by foreign host.