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Possible limitation of ngx_http_limit_req_module

May 11, 2015 10:59AM
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!

Best regards,
Jakub Wroblewski
Subject Author Posted

Possible limitation of ngx_http_limit_req_module

jwroblewski May 11, 2015 10:59AM

Re: Possible limitation of ngx_http_limit_req_module

Valentin V. Bartenev May 12, 2015 07:34AM

Re: Possible limitation of ngx_http_limit_req_module

jwroblewski May 12, 2015 09:25AM

Re: Possible limitation of ngx_http_limit_req_module

Valentin V. Bartenev May 12, 2015 09:44AM

Re: Possible limitation of ngx_http_limit_req_module

B.R. May 12, 2015 10:48AM

Re: Possible limitation of ngx_http_limit_req_module

Valentin V. Bartenev May 12, 2015 12:30PM

Re: Possible limitation of ngx_http_limit_req_module

jwroblewski May 12, 2015 12:33PM

Re: Possible limitation of ngx_http_limit_req_module

Valentin V. Bartenev May 12, 2015 01:02PM

Re: Possible limitation of ngx_http_limit_req_module

Maxim Dounin May 12, 2015 01:48PM



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