netdev - Re: [PATCH net-next] softirq: reduce latencies

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Message-ID: <1357252816.21409.26384.camel@edumazet-glaptop>
Date:	Thu, 03 Jan 2013 14:40:16 -0800
From:	Eric Dumazet <erdnetdev@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH net-next] softirq: reduce latencies

On Thu, 2013-01-03 at 22:08 +0000, Ben Hutchings wrote:
> On Thu, 2013-01-03 at 04:28 -0800, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > In various network workloads, __do_softirq() latencies can be up
> > to 20 ms if HZ=1000, and 200 ms if HZ=100.
> > 
> > This is because we iterate 10 times in the softirq dispatcher,
> > and some actions can consume a lot of cycles.
> > 
> > This patch changes the fallback to ksoftirqd condition to :
> > 
> > - A time limit of 2 ms.
> > - need_resched() being set on current task
> [...]
> > --- a/kernel/softirq.c
> > +++ b/kernel/softirq.c
> [...]
> > -#define MAX_SOFTIRQ_RESTART 10
> > +#define MAX_SOFTIRQ_TIME  min(1, (2*HZ/1000))
> [...]
> 
> Really?  Never iterate if HZ < 500?
> 

good catch, it should be max()


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