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

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Message-ID: <CADVnQynBPVr0qX2pu7FNwk6Y1BaW-pGf=JReJPCAj9RP=6t9_w@mail.gmail.com>
Date: Mon, 6 Jan 2025 21:50:02 -0500
From: Neal Cardwell <ncardwell@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Teodor Milkov <zimage@...soft.com>, netdev@...r.kernel.org, mst@...hat.com, 
	jasowang@...hat.com
Subject: Re: Download throttling with kernel 6.6 (in KVM guests)

On Mon, Jan 6, 2025 at 4:20 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 6 Jan 2025 22:15:37 +0200 Teodor Milkov 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.
>
> 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")

best,
neal