Re: Help with troubleshooting performance issue

On Tue, Apr 02, 2013 at 04:01:05PM +1100, Jafaruddin Lie wrote:
> Thanks, James.
> I am doing a lab comparing pptp performance and ipsec vpn
> performance, so this is not really for production :)

Doesn't seem interesting.

ipsec is entirely kernel based, so you will see a performance
improvement purely based on scheduling.

> I did tcpdump on the pptp server, I am seeing a few [TCP Previous
> Segment not captured] and [TCP ACKed unseen segment].
> Not sure what that means though.

Means the throughput degradation is due to missing packets.

> The machines are older machines, but they are dual-core machines.
> Assuming everything is optimised, are you saying that pptp might not
> be able to sustain more than around 6-8MBps traffic?

It depends on the CPU performance, the type of memory, the kernel
version, and the amount of contention among user-space processes.
What contention do you observe?  (uptime, load averages).

But yes, I would not be surprised if on a typical pair of machines
that PPTP might sustain a bandwidth of more than a tenth of native
link bandwidth.

You might isolate the problem further by testing a client or server
using another operating system.

There has been a kernel based PPTP implementation, but not in the
CentOS packaging.  You would probably need to patch and rebuild the
kernel in order to try that.

-- 
James Cameron
http://quozl.linux.org.au/

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