Re: [quicwg/base-drafts] ACK generation recommendation (#3304)

Ryan Hamilton <notifications@github.com> Tue, 17 December 2019 15:28 UTC

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From: Ryan Hamilton <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] ACK generation recommendation (#3304)
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On Tue, Dec 17, 2019 at 6:48 AM Nick Banks <notifications@github.com> wrote:

> Gorry raised the point that in experiments, this generates way too many
> ACK packets in high bandwidth networks, such as satellite networks. This
> has noticeable CPU costs for QUIC, for both sending as well as for
> receiving.
>
> Could you expand on this? I find this to be an extremely general
> statement, and not very helpful in understanding the real motivation for
> any possible changes. You say it "has noticeable CPU costs for QUIC", but
> are you referring to the client, server or some middle boxes somehow?
>
It can happen to either endpoint. Consider an HTTP/3 download. The client
ends up burning a bunch of CPU *sending* ACK, and the server a bunch of CPU
*processing* ACKs.

> Assuming you're worried about the CPU costs on the server side, has anyone
> explicitly measured the differences in CPU cost for generating different
> number of ACKs per RTT? What's the effect on FC? Fewer ACKs will mean the
> sender (I'm assuming the server?) is going to have to buffer more, and FC
> windows might be hit more often. Is this really such a big problem that is
> requires a spec change? In V1?
>
We've definitely noticed the CPU costs at Google on high
bandwidth connections with long flow. This led us to implement a variety of
ACK reduction strategies which reduced CPU use significantly. I think a
QUIC sender which sends ACKs of every 2 packets without and further limits
will be unable to take full advantage of the network and yes, we should
explicitly permit this in V1.

I don't believe we measured the impact on flow control. Our clients
typically advertise a very large flow control window which basically does
not get hit.

Cheers,

Ryan


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