netdev - Re: [net-next PATCH V2 1/9] net: frag evictor, avoid killing warm frag queues

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Date:	Fri, 30 Nov 2012 08:37:49 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	David Miller <davem@...emloft.net>, fw@...len.de,
	netdev@...r.kernel.org, pablo@...filter.org, tgraf@...g.ch,
	amwang@...hat.com, kaber@...sh.net, paulmck@...ux.vnet.ibm.com,
	herbert@...dor.hengli.com.au
Subject: Re: [net-next PATCH V2 1/9] net: frag evictor, avoid killing warm
 frag queues

On Fri, 2012-11-30 at 16:45 +0100, Jesper Dangaard Brouer wrote:
> On Fri, 2012-11-30 at 06:52 -0800, Eric Dumazet wrote:

> 
> > I dont know how you expect that many
> > datagrams being correctly reassembled with ipfrag_high_thresh=262144 
> 
> That's my point... I'm showing that its not possible, with out current
> implementation!

What I was saying is that the limits are too small, and we should
increase them for this particular need.

This has little to do with the underlying algo.

Assuming we have a hash table size of 1024 buckets, you could
easily add the following :

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 448e685..bc1bdf9 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -849,13 +849,13 @@ static inline void ip4_frags_ctl_register(void)
 static int __net_init ipv4_frags_init_net(struct net *net)
 {
 	/*
-	 * Fragment cache limits. We will commit 256K at one time. Should we
-	 * cross that limit we will prune down to 192K. This should cope with
+	 * Fragment cache limits. We will commit 4M at one time. Should we
+	 * cross that limit we will prune down to 3M. This should cope with
 	 * even the most extreme cases without allowing an attacker to
 	 * measurably harm machine performance.
 	 */
-	net->ipv4.frags.high_thresh = 256 * 1024;
-	net->ipv4.frags.low_thresh = 192 * 1024;
+	net->ipv4.frags.high_thresh = 4 << 20;
+	net->ipv4.frags.low_thresh = 3 << 20;
 	/*
 	 * Important NOTE! Fragment queue must be destroyed before MSL expires.
 	 * RFC791 is wrong proposing to prolongate timer each fragment arrival


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