On Monday 11 May 2015 10:59:46 jwroblewski wrote:
> Hi,
>
> I'm observing an inconsistent behavior of ngx_http_limit_req_module in nginx
> 1.7.12. The relevant excerpts from my config:
>
> http {
> ...
> # A fixed string used as a key, to make all requests fall into the same
> zone
> limit_req_zone test_zone zone=test_zone:1m rate=5r/s;
> ...
> server {
> ...
> location /limit {
> root /test
> limit_req zone=test_zone nodelay;
> }
> ...
> }
> }
>
> I use wrk to hammer the server for 5 secs:
>
> $ ./wrk -t 100 -c 100 -d 5 http://127.0.0.1/limit/test
> Running 5s test @ http://127.0.0.1/limit/test
> 100 threads and 100 connections
> Thread Stats Avg Stdev Max +/- Stdev
> Latency 2.82ms 2.96ms 15.12ms 88.92%
> Req/Sec 469.03 190.97 0.89k 62.05%
> 221531 requests in 5.00s, 81.96MB read
> Non-2xx or 3xx responses: 221506
> Requests/sec: 44344.69
> Transfer/sec: 16.41MB
>
> So, out of 221531 sent requests, 221506 came back with error. This gives
> (221531 - 221506) = 25 successful requests in 5 secs, so 5r/s, just as
> expected. So far so good.
>
> Now, what happens if I set rate=5000r/s:
>
> $ ./wrk -t 100 -c 100 -d 5 http://127.0.0.1/limit/test
> Running 5s test @ http://127.0.0.1/limit/test
> 100 threads and 100 connections
> Thread Stats Avg Stdev Max +/- Stdev
> Latency 3.64ms 5.70ms 36.58ms 87.43%
> Req/Sec 443.50 191.55 0.89k 65.04%
> 210117 requests in 4.99s, 77.38MB read
> Non-2xx or 3xx responses: 207671
> Requests/sec: 42070.61
> Transfer/sec: 15.49MB
>
> This time it is (210117 - 207671) = 2446 successful requests in 5 secs,
> which means 490r/s. Ten times lower then expected.
>
> I gathered some more figures, showing the number of 200 responses for
> growing value of "rate" parameter.
>
> rate=***r/s in zone cfg -- number of 200 responses
> 100 -- 87
> 200 -- 149
> 500 -- 344
> 1000 -- 452
> 10000 -- 468
> 100000 -- 466
>
> As you can see, the server keeps returning pretty much constant number of
> 200 responses once the "rate" parameter has surpassed 1000.
>
> I had a glimpse into the module's code, and this part caught my eye:
> https://github.com/nginx/nginx/blob/nginx-1.7/src/http/modules/ngx_http_limit_req_module.c#L402-L414.
> Basically, if consecutive requests hit the server in the same millisecond,
> the "ms = (ngx_msec_int_t) (now - lr->last)" part evaluates to 0, which sets
> "excess" to 1000, which is very likely to be greater than a "burst" value,
> which results in rejecting the request. This could also mean, that only the
> very first request hitting the server in given millisecond would be handled,
> which seems to be in line with the wrk test results, I've presented above.
>
> Please let me know if this makes sense to you!
>
[..]

Yes, the module is limited by millisecond timer resolution, but using so high
rate values without the burst parameter is meaningless.

So, just don't do that.

wbr, Valentin V. Bartenev

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