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Date:      Wed, 26 Sep 2007 17:00:57 +0200 (CEST)
From:      Oliver Fromme <olli@lurza.secnetix.de>
To:        freebsd-stable@FreeBSD.ORG, matrix@itlegion.ru
Subject:   Re: device polling and weird timer interrupt count from vmstat
Message-ID:  <200709261500.l8QF0vuI059492@lurza.secnetix.de>
In-Reply-To: <000f01c7ff9a$03b887e0$0c00a8c0@Artem>

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Artem Kuchin wrote:
 > Well, problem with top is that on dual 3GHZ box it alsway s
 > shows 0% load when not loaded with real traffic (web traffic) no matter
 > if it is polling of int handling.

Great, so your machine doesn't have any significant overhead
for the timer interrupt.  That was your question, wasn't it?

 > And when loaded with real traffic
 > web server eat a lot more cpu power then traffic handling, so, no
 > separate measurement of traffic cpu load is possible.

By "traffic cpu load" you mean the handling in the TCP/IP
stack and in the device driver, right?  That's always the
same, no matter whether polling is enabled or not.

In fact, with polling enabled you get _less_ interrupts,
so the overhead caused by the actual interrupt handling
is smaller.

 > Also, when it comes to public web server i can never be secure enough and
 > crazy load of traffic can come any time from DDOS attack which can bring
 > down any box. So, for public web server it is a matter of security and
 > managebility to have server interactive even on high traffic load. So, even from
 > this poing of view polling can be usefull.

Not really.  DDoS attacks against web servers usually work
on userland level, not on kernel level.  For example, a
simple way to perform such an attack would be to make many
requests so that your apache runs out of resources.

Polling does not help at all in that case.  Polling only
helps when the _kernel_ side is saturated with network
traffic, but that's usually not the case on a web server
where the Apache kicks the bucket much earlier than the
kernel.

Best regards
   Oliver

-- 
Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M.
Handelsregister: Registergericht Muenchen, HRA 74606,  Geschäftsfuehrung:
secnetix Verwaltungsgesellsch. mbH, Handelsregister: Registergericht Mün-
chen, HRB 125758,  Geschäftsführer: Maik Bachmann, Olaf Erb, Ralf Gebhart

FreeBSD-Dienstleistungen, -Produkte und mehr:  http://www.secnetix.de/bsd

"Being really good at C++ is like being really good
at using rocks to sharpen sticks."
        -- Thant Tessman



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