[j-nsp] Cut through and buffer questions

[Saku Ytti]

On Fri, 19 Nov 2021 at 10:49, james list <jameslist72 at gmail.com> wrote:

Hey,

> I try to rephrase the question you do not understand: if I enable cut through or change buffer is it traffic affecting ?

There is no cut-through and I was hoping after reading the previous
email, you'd understand why it won't help you at all nor is it
desirable. Changing QoS config may be traffic affecting, but you
likely do not have the monitoring capability to observe it.

> Regarding the drops here the outputs (15h after clear statistics):

You talked about MX, so I answered from MX perspective. But your
output is not from MX.

The device you actually show has exceedingly tiny buffers and is not
meant for Internet WAN use, that is, it does not expect significantly
higher sender rate to receiver rate with high RTT. It is meant for
datacenter use, where RTT is low and speed delta is small.

In real life Internet you need larger buffers because of this
senderPC => internets => receiverPC

Let's imagine an RTT of 200ms and receiver 10GE and sender 100GE.
- 10Gbps * 200ms = 250MB TCP window needed to fill it
- as TCP windows grow exponentially in absence of loss, you could have
128MB => 250MB growth
- this means, senderPC might serialise 128MB of data at 100Gbps
- this 128MB you can only send at 10 Gbps rate, rest you have to take
into the buffers
- intentionally pathological example
- 'easy' fix is, that sender doesn't burst the data at its own rate,
but does rate estimation and sends window growth at estimated receiver
rate, this practically removes buffering needs entirely
- 'easy' fix is not standard behaviour, but some cloudyshops configure
their linux like this thankfully (Linux already does bandwidth
estimation, and you can ask 'tc' to shape the session to esimated
bandwidth'

What you need to do is change the device to a one that is intended for
the application you have.
If you can do anything at all, what you can do, is ensure that you
have minimum amount of QoS classes and those QoS classes have maximum
amount of buffer. So that unused queues aren't holding empty memory
while used queue is starving. But even this will have only marginal
benefit.

Cut-through does nothing, because your egress is congested, you can
only use cut-through if egress is not congested.



-- 
  ++ytti