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From: Michael H. Warfield (mhw_at_wittsend.com)
Date: Thu 01 May 2003 - 14:54:03 BST

On Fri, May 02, 2003 at 01:29:42AM +1200, Sam Vilain wrote:
> On Tue, 29 Apr 2003 04:35, Michael H. Warfield wrote:
> > Actually, I also found it in chbind.c as well as in the kernel
> > patch. That's not real good having a dependency like that where two
> > numbers have to be maintained in sync like that. Might be better if
> > chbind could determine what the limit is from the kernel if they aren't
> > both dynamic. Including the value from the kernel header file is ugly
> > but extracting it dynamically via /proc (or an ioctl) seems like a bit
> > of overkill. Could have chbind be dynamic and run until it gets an
> > error back from the kernel indicating too many addresses...

> Really, maintaining a list of consecutive IP addresses is a misuse of the
> algorithm. It means that, for instance, when you try to bind to a port the
> kernel has to scan up to 16k (4096 addresses * 4 bytes each), which isn't
> terribly good. This is why the default limit is so low; keeps the overhead
> from this O(N) loop small.

        Woa! Who said anything about them being consecutive???
In fact, I think in my original message I mentioned assigning 64 or
more addresses to each vserver assigned psuedo randomly out of a pool
of 4094 (/20 - 2) addresses.

> Perhaps what you want could be achieved more efficiently by adding a
> netmask to the s_context structure, which defaults to, and
> applies only to the first IP address. It would be set via the set_ipv4root
> system call.

        Wouldn't help me. Even on my production colo virtual hosting
servers, that wouldn't help (much) since it then requires assignments on
netmask boundries. Ok, it does reduce the linear problem to a log base 2
problem so 16 slots would allow for 16 masked ranges but the ranges would
require more complex testing. Not sure where the cycle count break even
point would be. Would be a net loss in the "sparcely populated field"
example I'm dealing with.

> Of course it all depends on whether you care about losing a few thousand
> clock cycles every time you create a inet socket.


        The time loss problem should only be a problem when those number
of addresses are assigned. If you allocate space for 64 but only
assign 4 addresses, the kernel shouldn't search all 64 slots so
the computational issues are only a factor in those applications
taking advantage of the facility to that extent. OTOH... By having
a static structure like this, we are allocation kernel space memory
which is only rarely used (the additional space) for certain specific
applications. Perhaps it should be "run time definable" much along
the line of MAX_FILES or such. Set a sysctl or proc variable to
set the size.

> --
> Sam Vilain, sam_at_vilain.net

> Real computer scientists don't write code. They occasionally tinker
> with 'programming systems', but those are so high level that they
> hardly count (and rarely count accurately; precision is for
> applications.)


 Michael H. Warfield    |  (770) 985-6132   |  mhw_at_WittsEnd.com
  /\/\|=mhw=|\/\/       |  (678) 463-0932   |  http://www.wittsend.com/mhw/
  NIC whois:  MHW9      |  An optimist believes we live in the best of all
 PGP Key: 0xDF1DD471    |  possible worlds.  A pessimist is sure of it!

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