Hi,
On 09/11/17 18:08, Gert Doering wrote:
On Thu, Nov 09, 2017 at 05:19:14PM +0100, Jan Just Keijser wrote:
- without any load the ping times are ~ 7 ms , which is actually quite
good for ADSL
- when I run a long iperf session and then do a ping in the background,
the ping times go up to 800+ ms, then once the iperf is done, the ping
times go down again
It might be worth to experiment with --txqueuelen (on both sides)
or --tcp-queue-limit and --sndbuf (on the side that sends lots of data
into the TCP connection).
--tcp-queue-limit will limit the amount of data OpenVPN will try to
stuff into the TCP session, so will lead to some loss on the TCP-over-
TCP session, which *could* end up in slower sending by the file
transfer (etc.).
--sndbuf will limit the mount of data sitting inside the kernel for
outgoing packets. Reducing this to, say, 16000, will reduce achievable
throughput (because for througput, you want large buffers to really
sustain sending, not having to wait for the app to fill the buffer
while the network is idle), but will also reduce the amount of data
sitting "in front" of your pings -> better RTT.
I'd try "--sndbuf 16000 --tcp-queue-limit 8" for a start (on the sending
side) and see if that makes a noticeable difference. Then start tuning.
that definitely is worth experimenting with: I just re-ran the test and
in one direction I've got the latency spikes under control (ping time <
70 ms). This is achieved by adding "--sndbuf 32000" on the client and
"--rcvbuf 36000 --tcp-queue-limit 8" on the server. With this, download
speed is not affected yet "upstream ping" (i.e. the client pinging the
server) never exceeds 70 ms. During upload, the upstream ping now tops
off at ~ 400 ms, which is better than before, but not as significant as
the 700-> 70ms drop.
I'll keep experimenting - and thanks for pointing out those tweaking
parameters again for me ;)
HTH,
JJK
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