On Mon, 6 Jan 2025 21:50:02 -0500 Neal Cardwell wrote:

On Mon, Jan 6, 2025 at 4:20 PM Jakub Kicinski [off-list ref] wrote:

quoted

On Mon, 6 Jan 2025 22:15:37 +0200 Teodor Milkov wrote:  

quoted

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.  

Let's CC the virtio maintainers, then.

The fact that a 300ms sleep between connections makes the problem
go away is a bit odd from the networking perspective.

You may need to find a way to automate the test and try to bisect
it down :( This may help: https://github.com/arighi/virtme-ng  

IIUC, from Teodor's earlier message in the thread it sounds like he
was able to bisect the issue; he mentioned that git bisect traced the
regression to the commit:

    dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale")

My bad. I think I looked at it last week and couldn't figure out 
why the sleep make any difference.