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