On 5/17/2018 2:58 AM, Thomas Bellman wrote:
On 2018-05-17 02:41, Brian Rak wrote:
We're not even doing 10gbit of traffic, so the buffers should last at
least a little bit.
And you're not hitting 10 Gbit/s even under very short bursts of a few
milliseconds? Microbursts like that don't show up in "normal" usage
graphs where you only poll your switches/routers every minute or so.
It's possible that we have high traffic bursts, I don't really have the
data to say one way or another (we only graph traffic at 5 minute intervals)
Thanks for the tip about cut-through, we didn't have that enabled.
Do you happen to know if it works from a 10g port to a broken out
4x10g port?
Should do. From the perspective of the Trident II chip, they are
not any different from normal 10G ports. Cut-through doesn't work
between ports of different speed, and the ports involved must not
have any rate limiting or shaping, but other than that I don't know
of any limitations. (And if you receive broken packets, those will
be forwarded instead of thrown away; that is the only disadvantage
of cut-through mode that I have heard of.)
It's annoying to be dropping packets with a bunch of unused buffer
space.
Just make sure you don't fill your buffers so much that you get a long
(measured in time), standing queue, since that will just turn into a
long delay for the packets without helping anything (search for "buffer-
bloat" for mor information). Not a big problem on Trident II-based
hardware, but if you have equipment that boasts about gigabytes of buffer
space, you may need to watch out.
Oh, and I believe both changing buffer allocation and enabling/disabling
cut-through mode resets the Trident chip, causing a short period (less
than one second, I belive) where traffic is lost.
/Bellman
We've seen significant reductions in dropped traffic after changing the
buffer allocations. We're going to continue to investigate why it's
happening, but things seem to be much happier now.
I'm still not sure why the defaults are the way they are, I can't
imagine FCOE traffic is common enough to warrant inclusion in the
default configs.
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