Re: UDP stress-testing

[Alexander Aring <alex.aring@gmail.com>]

Hi Michael,

On Wed, Apr 13, 2016 at 08:50:22AM -0400, Michael Richardson wrote:
> 
> Alexander Aring <alex.aring@gmail.com> wrote:
>     > _All_ current supported transceivers have on hardware side _one_
>     > framebuffer only.
> 
>     > I would agree 3-4 frames should keep the transmitter buffer, because
>     > only one framebuffer, 250kBit (even slower on sub 1-GHz) and holding
>     > 3-4 skb's in background.
> 
> If we are counting frames rather than packets, then I agree that 3-4 is too
> low; we should accomodate 2-3 full-sized IPv6 packets worth of fragments.
> 1280 / 88 = 14 * 3 = 42.
> So I suggest somewhere between 32 and 48 as a good default.
> 
> Well, we really ought to use the BQL to get the right number!
>         https://lwn.net/Articles/469652/
> 

ok, I looked into one example to provide BQL for some ethernet driver
[0]. For SoftMAC we have for the flow-label stuff some helper
functions [1].

Maybe some point to start there and look what happens. :-)

>     > On most systems the qdisc default is pfifo (but I remember something
>     > that systemd changed to fq_codel as default).
> 
> yes, fq_codel is often the default now.
> 
>     > This will getting the queue full and with payload of 352 bytes it makes
>     > fragments invalid because pfifo will drop some which is part of the
>     > whole fragment.
> 
> I had assumed that the fragmentation happened after the qdisc.
> 
>     > What I know is, when we drop one fragment, then we could drop every
>     > fragment inside the queue which comes from the whole fragmented 6lowpan
>     > packet. it seems this will not be handled currently.
> 
> This is very important, and why I had assumed that fragmentation was
> afterwards.
> 

We could implement the following:

1. Splitting fragments
2. While splitting don't call dev_queue_xmit directly.
   Instead we have an own small queue where we first collect all fragments.
3. After all fragments are inside the queue we check if qdisc has the
   "free slots" to store all fragments then, e.g.
    if ((dev->tx_queue_len - qdisc_qlen(dev->qdisc) >= $FRAGMENTS)
    then
       for_each_skb_fragment
          dev_queue_xmit...
    else
       drop;

Something like that, but maybe I should look at first inside IPv6
fragmentation deeper. I think they handled such case somehow, too. If
not then IPv6 should handle it as well.

It should be a similar issue there and we can hopefully grab some code
from there.

- Alex

[0] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=4a89ba04ecc6377696e4e26c1abc1cb5764decb9
[1] http://lxr.free-electrons.com/source/net/mac802154/util.c#L22