Brutkey

Petr Tesařík
@ptesarik@infosec.exchange

@amonakov@mastodon.gamedev.place Interesting. Indeed, on a second thought, my explanation doesn't make much sense. But the observed reality is I can reliably get netperf throughput >= 1850 Mbps with CONFIG_RETHUNK=y but <= 1840 Mbps without (and with no other changes to the setup). All with tiny (64-byte) buffer size, so an extremely syscall-heavy workload.
EDIT: Obviously, the memory layout also changes, but I have checked that L1I cache misses are comparable (and approx. 0.1%) this time.

Petr Tesařík
@ptesarik@infosec.exchange

@amonakov@mastodon.gamedev.place But are you 100% certain that BPU cannot predict never-before-seen unconditional jumps?

Alexander Monakov
@amonakov@mastodon.gamedev.place

@ptesarik@infosec.exchange no, not 100% at all. Branch prediction needs to work in several stages, the initial stage needs to guess the next fetch block just on the basis of current fetch address, without inspecting the bytes. In the following cycles a more educated guess can resteer the prediction, but return address prediction also happens not so late in the frontend.

(also consider that address calculation for an uncond jump is "cheap" only if it doesn't cross a page, otherwise needs virt->phys address xlat)

Petr Tesařík
@ptesarik@infosec.exchange

@amonakov@mastodon.gamedev.place FWIW Intel documentation says:

Branches that do not have a history in the BTB are predicted using a static prediction algorithm.