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

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Date:	Thu, 28 Jan 2010 22:16:27 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	cold cold <nedkonedev@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: 0% cpu usasge after fresh boot or net restart but 10% CPU if 
 kernel flush route cache

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.

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