netdev - Re: [PATCH 00/16] Remove the ipv4 routing cache

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Message-ID: <1343406533.2626.13104.camel@edumazet-glaptop>
Date:	Fri, 27 Jul 2012 18:28:53 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	David Miller <davem@...emloft.net>, alexander.duyck@...il.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH 00/16] Remove the ipv4 routing cache

On Fri, 2012-07-27 at 09:23 -0700, Eric W. Biederman wrote:

> I am familiar.  But does hardware prefetching make a difference
> if your object is less than 64 bytes?
> 

Apparently yes, if the prefetch touches a dirtied neighbour cache line.

> I don't believe only allocating 64 bytes will be a problem,
> as no one else well be dirtying your cache line.
> 
> I suppose you could run into pathologies where your object
> is 3*64 bytes in size, but your expression doesn't handle
> that case either.
> 

Sure, but in most cases fib objects are under 128 bytes.


> The other alternative to guarantee very good cache behavior is
> to ensure you are allocating a power of two size up to some limit,
> perhaps page size.
> 

Good idea.

> My point is the magic 128 likely requires an explicatory comment and I
> think the net result is you have encoded something fragile that is good
> for testing but that will in the fullness of time do strange things that
> will be easy to overlook.

Sure, I'll send a v2, thanks.


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