Re: [PATCH v2] Proportional Rate Reduction for TCP.

[Nandita Dukkipati <nanditad@google.com>]

On Sat, Aug 20, 2011 at 5:41 AM, Ilpo Järvinen
<ilpo.jarvinen@helsinki.fi> wrote:
> On Fri, 19 Aug 2011, Nandita Dukkipati wrote:
>
>> Forgot to turn off gmail's rich formatting, so re-sending to the list.
>>
>> On Fri, Aug 19, 2011 at 3:25 AM, Ilpo Järvinen
>> <ilpo.jarvinen@helsinki.fi> wrote:
>> >
>> > On Fri, 19 Aug 2011, Nandita Dukkipati wrote:
>> >
>> > > +static void tcp_update_cwnd_in_recovery(struct sock *sk, int newly_acked_sacked,
>> > > +                                     int fast_rexmit, int flag)
>> > > +{
>> > > +     struct tcp_sock *tp = tcp_sk(sk);
>> > > +     int sndcnt = 0;
>> > > +     int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
>> > > +
>> > > +     if (tcp_packets_in_flight(tp) > tp->snd_ssthresh) {
>> > > +             if (WARN_ON(!tp->prior_cwnd))
>> > > +                     tp->prior_cwnd = 1;
>> >
>> > This should still be made larger to avoid problems if it ever will be
>> > needed.
>>
>> I am letting the value remain at 1, mainly because this is the valid
>> lowest non-zero value for snd_cwnd to take on. The main purpose of
>> this code is to catch any lurking bug outside of PRR which results in
>> an undesirable divide by 0 in PRR. I would like to fix that bug if I
>> find this code is executed.
>
> NACK, until this value is at least 2 * tp->snd_ssthresh. Or alternatively
> the fallback is removed so that we DBZ and do not end up wrecking the
> network.

If prior_cwnd is suspect at this point, snd_ssthresh cannot be trusted either.
So it appears to me that it's best to remove the fallback and spare
the network.
Done in patch v4.

Nandita

>
> Other than that I'm ok with the patch (assuming the branches I brought
> up earlier is ok for everybody else).
>
>
> --
>  i.