Re: [Bloat] [Cerowrt-devel] beating the drum for BQL

[Dave Taht <dave.taht@gmail.com>]

One of the things not readily evident in trying to scale up, is the
cost of even the most basic routing table lookup. A lot of good work
in this area landed in linux 4.1 and 4.2 (see a couple posts here:
https://vincent.bernat.im/en/blog/2017-performance-progression-ipv4-route-lookup-linux
)

Lookup time for even the smallest number of routes is absolutely
miserable for IPv6 -
https://vincent.bernat.im/en/blog/2017-ipv6-route-lookup-linux

I think one of the biggest driving factors of the whole TSO/GRO thing
is due to trying to get smaller packets through this phase of the
kernel, and not that they are so much more efficient at the card
itself. Given the kerfuffle over here (
https://github.com/systemd/systemd/issues/9725 ) I'd actually like to
come up with a way to move the linux application socket buffers to the
post-lookup side of the routing table. We spend a lot of extra time
bloating up superpackets just so they are cheaper to route.

TCAMs are expensive as hell, but the addition of even a small one,
readily accessible to userspace or from the kernel, might help in the
general case. I've actually oft wished to be able to offload these
sort of lookups into higher level algorithms and languages like
python, as a general purpose facility. Hey, if we can have giant GPUs,
why can't our cpus have tcams?

programmable TCAM support got enabled in a recent (mellonox?) product.
Can't find the link at the moment TCAMs of course, is where big fat
dedicated routers and switches shine, over linux - and even arp table
lookups are expensive in linux, though I'm not sure if anyone has
looked lately.