netdev - Re: [PATCH v4 net-next 1/1] sched: Add dualpi2 qdisc

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Message-ID: <ea2ccad9-6a4a-48e1-8e99-0289e13d501c@redhat.com>
Date: Tue, 29 Oct 2024 13:56:22 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: chia-yu.chang@...ia-bell-labs.com, netdev@...r.kernel.org,
 davem@...emloft.net, stephen@...workplumber.org, jhs@...atatu.com,
 edumazet@...gle.com, kuba@...nel.org, dsahern@...nel.org, ij@...nel.org,
 ncardwell@...gle.com, koen.de_schepper@...ia-bell-labs.com,
 g.white@...leLabs.com, ingemar.s.johansson@...csson.com,
 mirja.kuehlewind@...csson.com, cheshire@...le.com, rs.ietf@....at,
 Jason_Livingood@...cast.com, vidhi_goel@...le.com
Cc: Olga Albisser <olga@...isser.org>,
 Olivier Tilmans <olivier.tilmans@...ia.com>,
 Henrik Steen <henrist@...rist.net>, Bob Briscoe <research@...briscoe.net>
Subject: Re: [PATCH v4 net-next 1/1] sched: Add dualpi2 qdisc

On 10/22/24 00:12, chia-yu.chang@...ia-bell-labs.com wrote:
> +/* Default alpha/beta values give a 10dB stability margin with max_rtt=100ms. */
> +static void dualpi2_reset_default(struct dualpi2_sched_data *q)
> +{
> +	q->sch->limit = 10000;				/* Max 125ms at 1Gbps */
> +
> +	q->pi2.target = 15 * NSEC_PER_MSEC;
> +	q->pi2.tupdate = 16 * NSEC_PER_MSEC;
> +	q->pi2.alpha = dualpi2_scale_alpha_beta(41);	/* ~0.16 Hz * 256 */
> +	q->pi2.beta = dualpi2_scale_alpha_beta(819);	/* ~3.20 Hz * 256 */
> +
> +	q->step.thresh = 1 * NSEC_PER_MSEC;
> +	q->step.in_packets = false;
> +
> +	dualpi2_calculate_c_protection(q->sch, q, 10);	/* wc=10%, wl=90% */
> +
> +	q->ecn_mask = INET_ECN_ECT_1;
> +	q->coupling_factor = 2;		/* window fairness for equal RTTs */
> +	q->drop_overload = true;	/* Preserve latency by dropping */
> +	q->drop_early = false;		/* PI2 drops on dequeue */
> +	q->split_gso = true;

This is a very unexpected default. Splitting GSO packets earlier WRT the
H/W constaints definitely impact performances in a bad way.

Under which condition this is expected to give better results?
It should be at least documented clearly.

Thanks,

Paolo