400G forwarding - how does it work?
mohta at necom830.hpcl.titech.ac.jp
Sun Aug 7 09:13:11 UTC 2022
ljwobker at gmail.com wrote:
> Buffer designs are *really* hard in modern high speed chips, and
> there are always lots and lots of tradeoffs. The "ideal" answer is
> an extremely large block of memory that ALL of the
> forwarding/queueing elements have fair/equal access to... but this
> physically looks more or less like a full mesh between the
> memory/buffering subsystem and all the forwarding engines, which
> becomes really unwieldly (expensive!) from a design standpoint. The
> amount of memory you can practically put on the main NPU die is on
> the order of 20-200 **mega** bytes, where a single stack of HBM
> memory comes in at 4GB -- it's literally 100x the size.
I'm afraid you imply too much buffer bloat only to cause
unnecessary and unpleasant delay.
With 99% load M/M/1, 500 packets (750kB for 1500B MTU) of
buffer is enough to make packet drop probability less than
1%. With 98% load, the probability is 0.0041%.
But, there are so many router engineers who think, with
bloated buffer, packet drop probability can be zero, which
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