Re: Mbuf Clusters on 4.8

From: Bosko Milekic (bmilekic_at_technokratis.com)
Date: 06/25/03

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    Date: Wed, 25 Jun 2003 21:57:43 +0000
To: John B?ckstrand <sandos@home.se>

    On Thu, Jun 26, 2003 at 03:03:20AM +0200, John B?ckstrand wrote:
    > Ive been googling quite a bit now for problems with
    > running out of mbuf
    > clusters. Im basically sending a 30k datachunk down
    > 1000-4000 connections,
    > but 1000 is more than enough to quickly fill upp 8192
    > mbuf clusters. I also
    > tried setting maximum amount of mbuf clusters to 65536,
    > but that only made
    > the box hard-wire 86MB of 96MB RAM, making it just as
    > unsuable as a dead
    > machine.
    >
    > Of course, when the machine runs out of mbuf clusters,
    > it dies. I also found
    > this with google:
    >
    > "Finally, the fact that FreeBSD 3.x panics when it runs
    > out of
    > mbuf clusters is a well-known problem. The solution is
    > to not
    > let it run out of mbuf clusters by configuring a
    > sufficient
    > number for them."
    >
    > >From this it sounds as it is a problem that should be
    > fixed, but it
    > obviously isnt in 4.8. Is this behaviour now considered
    > acceptable? And if
    > so, doesnt this make FreeBSD extremely easy to kill
    > using a simple
    > DOS-attack? Is this "fixed" in any way on 5.1?
    >
    > ---
    > John B?ckstrand

      It's not panicking, it's running out of resources. Whenever you have
      this sort of problem you need to provide more information, there is
      absolutely no way I can help you like this. You need to, at the very
      minimum, give us 'netstat -m' output and make a serious attempt at
      figuring out what is consuming so many clusters. You could be running
      out of clusters but you could also be running out of memory before you
      run out of clusters, in which case you should probably _not_ increase
      nmbclusters and instead fix the underlying problem instead (re-work
      your application). In such a scenario, blindly bumping up nmbclusters
      can make the problem worse. Even if you had 'unlimited' (or
      dynamically growing) nmbclusters, you'd _still_ have the same problem
      and, what's more, it could actually render your system even more
      unusable as the machine would not be able to allocate memory for
      other more important uses. 

    --
Bosko Milekic  *  bmilekic@technokratis.com  *  bmilekic@FreeBSD.org
TECHNOkRATIS Consulting Services  *  http://www.technokratis.com/
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