netdev - Re: [PATCH net-next] mlx4: do not fire tasklet unless necessary

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Date:   Thu, 16 Feb 2017 15:13:54 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Saeed Mahameed <saeedm@....mellanox.co.il>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Matan Barak <matanb@...lanox.com>, jackm@...lanox.com
Subject: Re: [PATCH net-next] mlx4: do not fire tasklet unless necessary

On Thu, 2017-02-16 at 23:17 +0200, Saeed Mahameed wrote:

> so i guess you are not busy polling .. and adaptive moderation decided
> to lower down
> rx-usecs for you, and you are looking to improve latency.
> 
> > Interrupt moderation is a latency killer, we want our usec back.
> >
> 
> well, for RX we have adaptive moderation.
> and for TX we have xmit more.
> so interrupt moderation can be good if it is done right.
> 
> do i understand from this that you are against interrupt moderation ?

Absolutely against it. We prefer gro_flush_timeout.

The defaults values in mlx4 never were tuned for 40Gbit.

If you want to be able to reach 40Gbit on a single TCP flow, you want :

ethtool -C ethX rx-frames 16 rx-usecs-high 32

Or disable interrupt mitigation of course.


Interrupt moderation comes for free with NAPI really :

If under stress, number of interrupts will naturally decrease,
so what's the point with hardware _delaying_ an incoming packet
exactly ???

BTW interrupt moderation in mlx4 has at least two bugs.

I will send a patch.


> 
> >>
> >> > I wonder why this very expensive mb() is required, right before exiting
> >> > the interrupt handler.
> >>
> >> to make sure the HW knows we handled Completions up to (ci) consumer
> >> index. so it will generate next irq.
> >
> >
> > So why a mb() is needed exactly ?
> >
> > wmb() seems enough.
> >
> 
> Yes seems right, not sure why it is mb() though, i will have to check
> and get back to you on this.

Thanks.