netdev - Re: SACK scoreboard

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Message-ID: <20080109070318.GA8581@one.firstfloor.org>
Date:	Wed, 9 Jan 2008 08:03:18 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	David Miller <davem@...emloft.net>
Cc:	andi@...stfloor.org, jheffner@....edu, ilpo.jarvinen@...sinki.fi,
	lachlan.andrew@...il.com, netdev@...r.kernel.org,
	quetchen@...tech.edu
Subject: Re: SACK scoreboard

> It adds severe spikes in CPU utilization that are even moderate
> line rates begins to affect RTTs.
> 
> Or do you think it's OK to process 500,000 SKBs while locked
> in a software interrupt.

You can always push it into a work queue.  Even put it to
other cores if you want. 

In fact this is already done partly for the ->completion_queue.
Wouldn't be a big change to queue it another level down.

Also even freeing a lot of objects doesn't have to be
that expensive. I suspect the most cost is in taking
the slab locks, but that could be batched. Without
that the kmem_free fast path isn't particularly
expensive, as long as the headers are still in cache.

Long ago I had sk_buff optimized for the routing case so that freeing
can be all done with a single cache line. That is 
long gone, but could be probably restored.

But asking for protocol changes just to work around such a
internal implementation detail is ... <I miss words> 

> Perhaps you have another broken awk script to prove this :-)

Your hand waved numbers on inline sizes there were definitely worse 
than mine. 

-Andi

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