netdev - Re: 0% cpu usasge after fresh boot or net restart but 10% CPU if kernel flush route cache

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Message-ID: <41ac0f9e1001282338n76217590u61877f181c05dc06@mail.gmail.com>
Date:	Fri, 29 Jan 2010 09:38:31 +0200
From:	cold cold <nedkonedev@...il.com>
To:	netdev@...r.kernel.org
Subject: Re: 0% cpu usasge after fresh boot or net restart but 10% CPU if 
	kernel flush route cache

On Thu, Jan 28, 2010 at 11:16 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le jeudi 28 janvier 2010 à 20:49 +0200, cold cold a écrit :
>> what you mean drop packets ?
>>
>> i test 2 different things and shearing results with you
>> first test with high CPU is with garbage collection function
>> second results represent CPU usage with totally disabled  garbage collection
>
> ell, you didnt describe your benchmark method.
>
> 1) your results were on different rx/tx workload, and describing your
> workload is very important to be able to compare results. Then it should
> be exactly same workload.
>
> For example, when tx/tx load is high enough, less cpu overhead is spent
> on irq processing, since each IRQ delivers more packets per round.
>
> 2) you didnt sent "perf top" results for the second/last one.
>
>  But the first "perf top" results showed less than 1% of cpu time was
> used by cache cleanup. I guess you dont want to focus on this, since
> its already very good.
>
>
>
> Usually, when we want to bench a router, we study how it deals with DDOS
> workload. Feeding lot of packets to the device and study what percentage
> of them are actually transmitted. Goal being 100% of legit packets of
> course.
>
> Route cache settings matter in DDOS situations, and the flush operation
> can have a big impact on dropped frames because of cpu/ram congestion.
>
> Because of 600 seconds oscillations, its pretty hard to study exact cpu
> use of a router, unless taking samples on long periods.
>
>
>
>

I'm totally agree with you there must be some scheduler to release
route cache over the time.
So far i see garbage collector do this, but it cost a lot of CPU
probably its is a bug or design problem don't know
for this i make this 2 test to compare CPU usage with and without GC.


/ secret_interval 10 min,  1300000 route entries in cash ofter 10 min,
 7k new route on empty cache 2k on 1300000  /
all route cache parameters default. I try also with gc_elasticity from
8 to 2 and gc_interval from 60 to 1 but don't have
too much difference.

What I'm trying to say is that flush cash is almost instant ( less
then second on 1 CPU) so releasing of cash is not so heavy job
 ( you are right can have a big impact on dropped frames because of
cpu/ram congestion ) but my point is why GC need 5-6min
10% no  4 CPU to do same job ?
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