Quick performance deterioration when No. of clients increases

[Nikolaos Milas]

On 16/10/2013 1:32 μμ, Nikolaos Milas wrote:

> Now my problem is that there seems to be a limit of performance...
>
> Increasing stress load more than that (I am using tsung for load 
> testing), results only to increasing "error_connect_emfile" errors. 

I have been trying to resolve this behavior and I increased file 
descriptors to 400.000:

    # ulimit -n
    400000

since:

    # cat /proc/sys/fs/file-max
    592940

Now, I am running the following test: X number of users per sec visit 
the homepage and each one of them refreshes the page 4 times (at random 
intervals).

Although the test scales OK until 500 users per sec, then 
"error_connect_emfile" errors start again and performance deteriorates. 
See the attached comparative chart.

So, I have two questions:

 1. Is there a way we can tweak settings to make the web server scale
    gracefully up to the limit of its resources (and not deteriorate
    performance) as load increases? Can we leverage additional RAM (the
    box always uses up to 3.5 GB RAM, despite the load, and despite the
    fact that the VM now has 6 GB)?
 2. If not, how can we safeguard the web server by setting a suitable
    limit which cannot be surpassed to cause performance deterioration?

Please advise.

Thanks and regards,
Nick
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