netdev - Re: [patch] pktgen: bug when calling ndelay in x86 architectures

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Date:	Tue, 18 Oct 2011 13:56:47 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Turull <daniel.turull@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Robert Olsson <robert@...julf.net>,
	Voravit Tanyingyong <voravit@....se>,
	Jens Laas <jens.laas@...m.uu.se>
Subject: Re: [patch] pktgen: bug when calling ndelay in x86 architectures

Le mardi 18 octobre 2011 à 13:08 +0200, Daniel Turull a écrit :
> The value selected to delay the transmission in pktgen with the ndelay function should be lower.
> In Linux/arch/x86/include/asm/delay.h and Linux/arch/sh/include/asm/delay.h
> the maximal expected value for a constant is 20000 ns.
> 
> Signed-off-by: Daniel Turull <daniel.turull@...il.com>
> ---
> diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> index 796044a..e17bd41 100644
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -2145,7 +2145,7 @@ static void spin(struct pktgen_dev *pkt_dev, ktime_t spin_until)
>  	}
>  
>  	start_time = ktime_now();
> -	if (remaining < 100000)
> +	if (remaining < 20000)
>  		ndelay(remaining);	/* really small just spin */
>  	else {
>  		/* see do_nanosleep */

But 'remaining' is not a constant.

If we want exactly 40.000 packets per second rate (25 us between
packets), your patch makes this not quite possible without
CONFIG_HIGH_RES_TIMERS and probable high jitter because of scheduler
effects.

pktgen is kind of special, we _want_ a cpu for our exclusive use.



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