# Gzip - unexplained side effects **URL:** \ **Category:** NGINX\ **Created:** [November 9, 2011, 3:14am UTC](https://www.ruby-forum.com/t/gzip-unexplained-side-effects/213692 "2011-11-09T03:14:48Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![luislavena](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/0ea827/32.png) [@luislavena](https://www.ruby-forum.com/u/luislavena)\ **Post date:** [November 9, 2011, 3:14am UTC](https://www.ruby-forum.com/t/gzip-unexplained-side-effects/213692/1 "2011-11-09T03:14:48Z")
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: > **[gzip - unexplained side effects](https://forum.nginx.org/read.php?2,218044,218044#msg-218044)** > > 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... ---
**Author:** ![dbanks](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c68b51/32.png) [@dbanks](https://www.ruby-forum.com/u/dbanks)\ **Post date:** [November 9, 2011, 12:56pm UTC](https://www.ruby-forum.com/t/gzip-unexplained-side-effects/213692/2 "2011-11-09T12:56:49Z")
On Nov 8, 2011, at 9:14 PM, dbanks wrote: > gzip\_comp\_level 9; Not really related to your question, but this uses much more cpu than the default for very little gain. We always run this at “1” Unless you have some special requirements (with benchmarks), I’d do the same. –Brian ---
**Author:** ![dbanks](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c68b51/32.png) [@dbanks](https://www.ruby-forum.com/u/dbanks)\ **Post date:** [November 9, 2011, 1:42pm UTC](https://www.ruby-forum.com/t/gzip-unexplained-side-effects/213692/3 "2011-11-09T13:42:03Z")
On Tue, Nov 08, 2011 at 09:14:04PM -0500, dbanks wrote: > incoming requests decreased a bit: as if some requests were being > > ``` > #gzip_min_length 1100; #if it fits in one packet, no worries > #gzip_http_version 1.1; > > keepalive_timeout 6; > keepalive_requests 4; > > ``` If you comment gzip\_buffers, you see the previous site state ? What is typical uncompressed and compressed response size ? The default gzip\_buffers are “32 4k”, so they can keep up to 128K. And as it was already suggested it’s better to use default gzip\_comp\_level 1. – Igor S. ---
**Author:** ![dbanks](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c68b51/32.png) [@dbanks](https://www.ruby-forum.com/u/dbanks)\ **Post date:** [November 9, 2011, 8:09pm UTC](https://www.ruby-forum.com/t/gzip-unexplained-side-effects/213692/4 "2011-11-09T20:09:48Z")
Hi Igor, thanks for your response! > > If you comment gzip\_buffers, you see the previous site state ? If I comment gzip\_buffers, gzip\_min\_length, and gzip\_http\_version, I see the previous (desired) behavior. > > What is typical uncompressed and compressed response size ? Responses are 6.8k or smaller uncompressed, 2.6k compressed (gzip\_comp\_level=9), 2.8k compressed (gzip\_comp\_level=1). About half of the responses are this size, and the other half are less than 1k uncompressed. We currently have more than adequate CPU and want to minimize bandwidth costs, so I had assumed that more compression was better. Is there another reason that I should stay with the default gzip\_comp\_level=1? (I’m happy to try it–just curious.) Cheers, Dean Posted at Nginx Forum: [http://forum.nginx.org/read.php?2,218044,218117#msg-218117](http://forum.nginx.org/read.php?2,218044,218117#msg-218117) ---
**Author:** ![dbanks](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c68b51/32.png) [@dbanks](https://www.ruby-forum.com/u/dbanks)\ **Post date:** [November 18, 2011, 5:51pm UTC](https://www.ruby-forum.com/t/gzip-unexplained-side-effects/213692/5 "2011-11-18T17:51:35Z")
I spent more time testing this particular issue. I believe that what appeared to be lost traffic is simply due to the shortened keepalives and the load balancer favoring keepalive connections over new connections. However, there still seems to be a link between gzip settings and the number of open connections (keepalives). gzip on; gzip\_comp\_level 1; gzip\_types text/javascript text/plain application/x-javascript; gzip\_disable “MSIE [1-6].(?!.\*SV1)” #gzip\_buffers 64 4k; #gzip\_min\_length 1100; #if it fits in one packet, no worries #gzip\_http\_version 1.1; I have confirmed that if any one of the three config lines at the bottom (the ones that are commented out) are present, the number of open connections drops from ~7600 to ~1300. Removing that line from the config and reloading restores normal operation. Strange. Cheers, Dean Posted at Nginx Forum: > **[Re: gzip - unexplained side effects](https://forum.nginx.org/read.php?2,218044,218575#msg-218575)** > > 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... ---
**Author:** ![dbanks](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c68b51/32.png) [@dbanks](https://www.ruby-forum.com/u/dbanks)\ **Post date:** [November 18, 2011, 6:40pm UTC](https://www.ruby-forum.com/t/gzip-unexplained-side-effects/213692/6 "2011-11-18T18:40:26Z")
Hello! On Fri, Nov 18, 2011 at 11:50:53AM -0500, dbanks wrote: > I spent more time testing this particular issue. I believe that what > appeared to be lost traffic is simply due to the shortened keepalives > and the load balancer favoring keepalive connections over new > connections. However, there still seems to be a link between gzip > settings and the number of open connections (keepalives). > > gzip on; > gzip\_comp\_level 1; > gzip\_types text/javascript text/plain application/x-javascript; > gzip\_disable “MSIE [1-6].(?!.\*SV1)” It looks like you’ve missed “;” here. This is probably a reason for all “strange” effects you observe - missing “;” causes “gzip\_disable” to eat next directive in config. > #gzip\_buffers 64 4k; > #gzip\_min\_length 1100; #if it fits in one packet, no worries > #gzip\_http\_version 1.1; > > I have confirmed that if any one of the three config lines at the bottom > (the ones that are commented out) are present, the number of open > connections drops from ~7600 to ~1300. Removing that line from the > config and reloading restores normal operation. Strange. Maxim D. ---
**Author:** ![dbanks](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c68b51/32.png) [@dbanks](https://www.ruby-forum.com/u/dbanks)\ **Post date:** [November 18, 2011, 8:01pm UTC](https://www.ruby-forum.com/t/gzip-unexplained-side-effects/213692/7 "2011-11-18T20:01:23Z")
Hello Maxim! Mystery solved. I cannot believe that I missed that! Cheers, Dean Posted at Nginx Forum: > **[Re: gzip - unexplained side effects](https://forum.nginx.org/read.php?2,218044,218580#msg-218580)** > > 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...