netdev - Re: Fwd: Linux stack performance drop (TCP and UDP) in 3.10 kernel in routed scenario

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Date:	Wed, 04 Jun 2014 07:59:53 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Suprasad Mutalik Desai <suprasad.desai@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.ne
Subject: Re: Fwd: Linux stack performance drop (TCP and UDP) in 3.10 kernel
 in routed scenario

On Wed, 2014-06-04 at 19:23 +0530, Suprasad Mutalik Desai wrote:

> I understood from you that the old route cache mechanism had DoS
> vulnerabilities, so the new mechanism is implemented. What I am trying
> to figure out is whether that will cause the kind of throughput drop
> that I am seeing ?
> 
> TCP performance
>             - Upstream : 140 Mbps(Linux 2.6.32) --> 101Mbps (Linux 3.10.12)
>             - Downstream : 148 Mbps(Linux 2.6.32) --> 106Mbps (Linux 3.10.12)

Sure.

A cache is supposed to help performance, right ?

Problem with IPv4 route cache is that is was too easy to flood it and
get worse performance.

It had simply huge memory costs, and non predictable behavior [1]

Better have a system behaving correctly at 99 percentile,
than a system working well _only_ if workload is very gentle.

[1] Well, sort of : prediction was : it's so easy to remotely crash the
host.

Your host has very little cache on cpu, very little bandwidth, so the
previous IPv4 cache was probably helping.

Its yet not clear why a router has to checksum TCP packets.

Maybe a conntracking requirement...

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