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From David Boreham <>
Subject Re: accepting large number of inbound TCP connections
Date Fri, 29 Dec 2017 19:44:51 GMT
I should say that I don't know much about ATS but I have spent some time 
looking into similar problems with other servers over the years. Some 
ideas below:

On 12/29/2017 3:56 AM, Mateusz Zajakala wrote:
> CPU utilization does not exceed 40% during peak traffic. I also 
> checked the number of sockets in connection
Note that 40% aggregate CPU on a many-core system can easily hide a 
saturated single thread. If under your workload the server ends up doing 
much work in a single thread, that can starve overall throughput. e.g. 
on your 8-core box one thread maxing out a core would only show up as 
12.5% -- obviously lower than your observed 40%.
> pending state (SYN_RECV) and it never goes above 20, so I suppose 
> accepting incoming connections is not the bottleneck.
> What about the number of worker threads? I'm using autoconfig with 
> default scale factor (1.5) which on my system (8 cores) creates 27 
> threads for traffic_server. Does it make sense to increase the scale 
> factor if my CPU utilization is not high? will this improve the 
> overall performance? What about stacksize?
I would recommend first gathering some data along the lines of "ok, so 
what _is_ it doing?" rather than theorizing about solutions. For example 
use "pstack", or a similar tool to snapshot the ATS process' thread 
stacks at full-load. Take a few such samples and look at them to see 
what it is up to. If you see for example all the threads busy doing work 
then that might be good supporting evidence for making a thread pool 
larger. or, is the accept thread always running (indicating the incoming 
accept workload has saturated one core). I suspect there are various 
counters and such that will be maintained by the ATS code and can be 
inspected on a live server -- typically these will give you some idea 
what is happening (e.g. work is queuing up waiting on threads).

A good way to think through a problem like this is to try to imagine 
what the server should be doing under the load you have. Once you have 
that mental picture, go look at what it is actually doing and see what's 
> How should I go on about finding the cause of some of the clients not 
> being able to connect occasionally?

See if you can reproduce the problem yourself with a test client (e.g. 
curl/wget). If you can then good : now work to "trace" what is happening 
with the packets from that client. You can use a netfilter/tcpdump 
filter to target only its IP or MAC to isolate the traffic you want to 
look at vs the deluge with low overhead. This should tell you if the 
stall is occurring at the NIC or in the kernel or in user space. To dig 
into what's going on in user space use logging (I assume but don't know 
for sure that ATS can be made to log the client IP). If you need more 
information to debug than existing logging will give you : add new code 
to log useful information for your investigation.

If you can't reproduce the issue with your own client, well that's not 
great, but you can attempt to work "backwards" to a reproduced case by 
capturing all or a decent sample of the network traffic then analyzing 
it statically to find examples.

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