On Mon, Jan 6, 2025 at 3:15 PM Teodor Milkov [off-list ref] wrote:

Hello,

Following up on my previous email, I’ve found the issue occurs
specifically with the |virtio-net| driver in KVM guests. Switching to
the |e1000| driver resolves the slowdown entirely, with no throttling in
subsequent downloads.

The reproducer and observations remain the same, but this detail might
help narrow down the issue.

Thanks for narrowing it down! Interesting. So this sounds like
something specific about the way the virtio-net driver is handling
receive buffer memory, and the way that is interacting with Eric's
excellent recent commit to make receive window management more
precise:

dfa2f0483360 tcp: get rid of sysctl_tcp_adv_win_scale

best regards,
neal