gzip - unexplained side effects

[dbanks]

Hi,

I'm running nginx 1.0.0 in front of a FCGI backend.  We've been running
in production for about 4 months, and have really been impressed with
the performance and stability of nginx.

We run a medium-volume appliction: 1000 to 4000 requests/sec spread over
2 instances by an upstream round robin load balancer.  We use
keepalives, but keep them rather short given the request volume to keep
the number of open connections manageable.  

Recently, I was working to improve our gzip settings.  The largest
change was that I added an explicit gzip buffer line (find my config
below - the three lines that are commented out seem to be correlated
with this issue)

After the change, the volume of outbound traffic decreased measurably,
as if gzip was not originally doing much due to inadequate buffer space.
 Great news!, or so I thought.  What also changed was that the number of
active connections fell by about 75% - the gzip change was somehow
causing keepalives to be closed prematurely.  Also, our volume of
incoming requests decreased a bit: as if some requests were being
aborted (though accepts == handled).   The request volume makes
debugging this particular issue somewhat troublesome, since I have yet
to replicate it in a quiet instance.

The guts of my configuration appear below.  This is such an unexpected
issue that I'm not doing a great job of setting up my question well. 
What I think I'd like to know is how could a change to the gzip buffers
(or the other two commented changes) impact keepalives or overall
connection negotiation?  Also, any suggestions as to how to go about
debugging it?

    sendfile        off;  
    tcp_nodelay on;  
    ignore_invalid_headers  on;  
    if_modified_since off;

    gzip on;
    gzip_comp_level 9;
    gzip_types text/javascript text/plain application/x-javascript;
    gzip_disable "MSIE [1-6]\.(?!.*SV1)"  
    #gzip_buffers 512 4k;
    #gzip_min_length  1100;  #if it fits in one packet, no worries
    #gzip_http_version 1.1;

    keepalive_timeout  6;  
    keepalive_requests 4;  

Cheers,

Dean

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,218044,218044#msg-218044