Nginx throttling issue?

[John Melom]

Maxim,

Thank you for your reply.  I will look to see if "netstat -s" detects any listen queue overflows.

John


-----Original Message-----
From: nginx [mailto:nginx-bounces at nginx.org] On Behalf Of Maxim Dounin
Sent: Tuesday, March 27, 2018 6:55 AM
To: nginx at nginx.org
Subject: Re: Nginx throttling issue?

Hello!

On Mon, Mar 26, 2018 at 08:21:27PM +0000, John Melom wrote:

> I am load testing our system using Jmeter as a load generator.
> We execute a script consisting of an https request executing in a
> loop.  The loop does not contain a think time, since at this point I
> am not trying to emulate a “real user”.  I want to get a quick look at
> our system capacity.  Load on our system is increased by increasing
> the number of Jmeter threads executing our script.  Each Jmeter thread
> references different data.
>
> Our system is in AWS with an ELB fronting Nginx, which serves as a
> reverse proxy for our Docker Swarm application cluster.
>
> At moderate loads, a subset of our https requests start experiencing
> to a 1 second delay in addition to their normal response time.  The
> delay is not due to resource contention.
> System utilizations remain low.  The response times cluster around 4
> values:  0 millilseconds, 50 milliseconds, 1 second, and 1.050
> seconds.  Right now, I am most interested in understanding and
> eliminating the 1 second delay that gives the clusters at 1 second and
> 1.050 seconds.
>
> The attachment shows a response time scatterplot from one of our runs.
> The x-axis is the number of seconds into the run, the y-axis is the
> response time in milliseconds.  The plotted data shows the response
> time of requests at the time they occurred in the run.
>
> If I run the test bypassing the ELB and Nginx, this delay does not
> occur.
> If I bypass the ELB, but include Nginx in the request path, the delay
> returns.
>
> This leads me to believe the 1 second delay is coming from Nginx.

There are no magic 1 second delays in nginx - unless you've configured something explicitly.

Most likely, the 1 second delay is coming from TCP retransmission timeout during connection establishment due to listen queue overflows.  Check "netstat -s" to see if there are any listen queue overflows on your hosts.

[...]

--
Maxim Dounin
http://mdounin.ru/
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