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

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Date:	Wed, 4 Jun 2014 23:11:05 +0530
From:	Suprasad Mutalik Desai <suprasad.desai@...il.com>
To:	Eric Dumazet <eric.dumazet@...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

Hi Eric,


On Wed, Jun 4, 2014 at 8:29 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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.
>

Yes , you are correct . We are working on an embedded router which has
limited cache .

So, could you please suggest on how to address this topic w.r.t
embedded devices ( routers) which has limited memory resources ?

> Its yet not clear why a router has to checksum TCP packets.
>
> Maybe a conntracking requirement...
>

Yes, in our case the checksum is done in Linux stack as NIC doesn't
support checksum offloading .
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