Unexpected result with limit_req module

[Maxim Dounin]

Hello!

On Sun, Sep 20, 2015 at 05:51:26PM +0800, flygoast wrote:

> Hi, all,
> 
> 
> I use limit_req module to limit QPS to my upstream, conf like this:
> http {
> limit_req_zone $request_uri zone=req_one:10m rate=10000r/s;
> server {
> ...
> location / {
> limit_req zone=req_one;
>        }
> }
> }
> 
> 
> I use benchmarking tool to stress on one url, but only about 1000 request. 
> 
> 
> I checked source code, found that:
>             ms = (ngx_msec_int_t) (now - lr->last);
> 
> 
>             excess = lr->excess - ctx->rate * ngx_abs(ms) / 1000 + 1000;
> 
> 
>             if (excess < 0) {
>                 excess = 0;
>             }
> 
> 
>             *ep = excess;
> 
> 
>             if ((ngx_uint_t) excess > limit->burst) {
>                 return NGX_BUSY;
>             }
> 
> 
> At here, ms can be '0', so in a millisecond, only the first 
> request can pass the checking. After I set burst value to (rate 
> / 1000) in configuration file, the QPS result is expected.
> 
> 
> I'm not sure whether this is a bug or It's my fault in configuration.

That's expected behaviour.  As nginx tracks time with millisecond 
resolution, two requests within the same millisecond essentially 
mean infinite rate - unless nginx can pass these requests and 
count them on a longer period of time.  That is, if you use high 
rates, you have to configure burst as well.

-- 
Maxim Dounin
http://nginx.org/