netdev - Re: Download throttling with kernel 6.6 (in KVM guests)

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Message-ID: <CADVnQyntg=3ugLxr+tdyb0A+=KDkVGkGPJVvFTOg0XpoyMSQEw@mail.gmail.com>
Date: Mon, 6 Jan 2025 16:15:15 -0500
From: Neal Cardwell <ncardwell@...gle.com>
To: Teodor Milkov <zimage@...soft.com>
Cc: netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>
Subject: Re: Download throttling with kernel 6.6 (in KVM guests)

On Mon, Jan 6, 2025 at 3:15 PM Teodor Milkov <zimage@...soft.com> 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