Re: Mbuf Clusters on 4.8

From: John Bäckstrand (sandos_at_home.se)
Date: 06/26/03

  • Next message: John Bäckstrand: "Re: Mbuf Clusters on 4.8" 
    To: "Bosko Milekic" <bmilekic@technokratis.com>
Date: Thu, 26 Jun 2003 13:05:04 +0200

    > 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.

    Yes, you are right, I didnt get a panick. Firstly,
    heres is /var/log/messages from when the box hung:

    Jun 26 02:29:30 sandbsd /kernel: All mbuf clusters
    exhausted, please see tuning(7).
    Jun 26 02:29:35 sandbsd last message repeated 4 times
    Jun 26 02:29:35 sandbsd /kernel: rl0: watchdog timeout
    Jun 26 02:29:36 sandbsd /kernel: All mbuf clusters
    exhausted, please see tuning(7).
    Jun 26 02:29:56 sandbsd last message repeated 17 times
    Jun 26 02:29:57 sandbsd /kernel: rl0: watchdog timeout

    I cant give netstat -m after it hung, obviously, but
    with 8192 max mbuf clusters I can see some time before
    the hang that peak is at 8192 clusters, I also saw lots
    of requests for memory denied, and requests for memory
    delayed, but no calls to drain routines. I also saw
    about 20MB allocated to network (60% of mb_map). This
    was the last time I tried hanging it, and I somehow
    thought it mas managing. After all, denying requests
    means it did actually cope with a memory starvation.
    The NIC is a realtek 8139. I wonder what significance
    those watchdog timeout messages have? I might try with
    a 3com later today (any card which doesnt require mbufs
    should be finer, if there is any such thing). 

    ---
John Bäckstrand
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