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by secondcoming 572 days ago
I looked at using DPDK on some of our GCP instances but it requires setting up a second VPC, which was one hurdle too much.

I’m hoping that io_uring makes all of this unnecessary anyway.

I recall reading a paper where someone noticed that for every packet the Linux kernel receives it has to check if any application has opened a raw socket. Raw sockets are initially needed to allow DHCP to work, so once your machine has been assigned an IP address you can (probably) turn this service off and so give the kernel less work to do. (My memory of the exact details may be sketchy).
2 comments
Polizeiposaune 572 days ago
DHCP issues address leases, not permanent assignments; leases have an expiration time (and earlier suggested renewal/rebind times). So the DHCP client must periodically renew -- if the tenant doesn't renew (perhaps because the DHCP client has been disabled), the DHCP service may lease the address to another tenant.

If the DHCP server hasn't moved to a new address this renewal can be done over unicast using the leased address - however, if the client doesn't receive a response from the server the client state machine will eventually discard the leased address and fall back to broadcast with an all-zeros source address (which is presumably what requires a raw socket).

The DHCP client implementation in question likely keeps the raw socket open for potential future use in this case. A client might be able to close the raw socket and reopen it later (but security folks might also want it to drop the privilege required to reopen the raw socket, and it might be hard to have an ironclad guarantee that the raw socket can be reopened later on a machine that's short on free kernel memory..).

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secondcoming 572 days ago
Not on GCP's GCE at least
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Matthias247 572 days ago
io_uring reduces the overhead of system calls - but it doesn't do anything to reduce the overhead of the actual networking stack.

If your send/receive calls spend most CPU time in going through routing/fragmentation/filter/BPF/etc path in the networking stack, then uring (or other APIs which just reduce the system call overhead, like SendMmsg/Recvmmsg for UDP) might only make a small difference. Source: Lots of profiling while implementing QUIC libraries.

An alternative to DPDK that allows to bypass the kernel networking stack would be AF_XDP.

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