200ms delay when serving stale content and proxy_cache_background_update enabled

[Jean-Paul Hemelaar]

I think this solves the issue: http://hg.nginx.org/nginx/rev/9552758a786e

Thanks,

JP

On Wed, Mar 15, 2017 at 11:05 AM, Jean-Paul Hemelaar <hemelaar at desikkel.nl>
wrote:

> Hi,
>
> I noticed a delay of approx. 200ms when the proxy_cache_background_update
> is used and Nginx sends stale content to the client.
>
> Current setup:
> - Apache webserver as backend sending a slow response delay.php that
> simply waits for 1 second: <?php usleep(1000000); ?>
> - Nginx in front to cache the response, and send stale content it the
> cache needs to be refreshed.
> - wget sending a request from another machine
>
> Nginx config-block:
>     location /delay.php {
>      proxy_pass  http://backend;
>      proxy_next_upstream error timeout invalid_header;
>      proxy_redirect http://$host:8000/ http://$host/;
>      proxy_buffering on;
>      proxy_connect_timeout 1;
>      proxy_read_timeout 30;
>      proxy_cache_background_update on;
>
>      proxy_http_version 1.1;
>      proxy_set_header Connection "";
>
>      proxy_cache            STATIC;
>      proxy_cache_key        "$scheme$host$request_uri";
>      proxy_cache_use_stale  error timeout invalid_header updating http_500
> http_502 http_503 http_504;
>      proxy_set_header        Host            $host;
>      proxy_set_header        X-Real-IP       $remote_addr;
>      proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
>      proxy_set_header        Accept-Encoding  "";
>
>      # Just to test if this caused the issue, but it doesn't change
>      tcp_nodelay on;
>    }
>
> Wget request: time wget --server-response --output-document=/dev/null "
> http://www.example.com/delay.php?teststales=true"
> Snippet of wget output: X-Cached: STALE
> Output of time command: real 0m0.253s
>
> Wget request: time wget --server-response --output-document=/dev/null "
> http://www.example.com/delay.php?teststales=true"
> Snippet of wget output: X-Cached: UPDATING
> Output of time command: real 0m0.022s
>
> So a cache HIT (not shown) or an UPDATING are fast, sending a STALE
> response takes some time.
> Tcpdump showed that all HTML content and headers are send immediately
> after the request has been received, but the last package will be delayed;
> that's why I tested the tcp_nodelay option in the config.
>
> I'm running version 1.11-10 with the patch provided by Maxim:
> http://hg.nginx.org/nginx/rev/8b7fd958c59f
>
> Any idea's on this?
>
> Thanks,
>
> Jean-Paul
>
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