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SubjectRe: Annoying /proc/net/dev rollovers.
On Mon, Feb 17, 2003 at 08:58:40PM -0800, David Lang wrote:
> don't forget that 10G ethernet is starting to leak out of the labs into
> the real world. I don't know of any linux support yet, but it will come
> and then you will be able to overflow 32bit bitcounters multiple times per
> second.

A machine capable to support full data speed of 10G ether needs ...
around 1.3 GB/sec I/O speed both ways for the card, which at
64-bit PCI-X 533 -- is at most 4.3 GB/sec. In reality one can't
quite get the theorethical maximum out of the hardware.
One full-speed full-duplex 10G ether is barely doable with that new
version of PCI-X.

A giga-ether interface (or two) can be done in current generation
hardware, and even some usefull things can be done to fill the pipe.

I do suppose that at the time we are also using 64-bit processors,
in which incrementing 64-bit counter variables uninterruptably is

I leave it as a thought excercise, as to why non-irq-blocking spinlock
is not a good idea to ensure data update monotonicity.

There are algorithmic ways to handle interruptible two-fetch
consistency problem in current 32-bit hardware. None of those
are being used, as far as I know:

add to less-significant-long
add carry to more-significant-long

reader context:
read less-significant-long into ax
read more-significant-long into bx
compare less-significant-long with ax
if differ, start from begin
compare more-significant-long with bx
if differ, start from begin
return ax,bx

That way the reader need not worry interrupting,
but implementation is -- likely -- assembly.

No spinlocks, no irq-blocking...

> David Lang

/Matti Aarnio

> On Mon, 17 Feb 2003, Matti Aarnio wrote:
> > Date: Mon, 17 Feb 2003 12:35:53 +0200
> > From: Matti Aarnio <>
> > To: Mark J Roberts <>,
> > Subject: Re: Annoying /proc/net/dev rollovers.
> >
> > On Sun, Feb 16, 2003 at 08:21:56PM -0800, Chris Wedgwood wrote:
> > > On Sun, Feb 16, 2003 at 08:46:05PM -0600, Mark J Roberts wrote:
> > > > When the windows box behind my NAT is using all of my 640kbit/sec
> > > > downstream to download movies, it takes a little over 14 hours to
> > > > download four gigabytes and roll over the byte counter.
> > >
> > > Therefore userspace needs to check the counters more often... say ever
> > > 30s or so and detect rollover. Most of this could be simply
> > > encapsulated in a library and made transparent to the upper layers.
> >
> > Some of my colleques complained once, that at full tilt
> > the fiber-channel fabric overflowed its SNMP bitcounters
> > every 2 seconds.
> >
> > "we need to do polling more rapidly, than the poller can do"
> >
> > The SNMP pollers do handle gracefully 32-bit unsigned overlow,
> > they just need to get snapshots in increments a bit under 2G...
> > (Hmm.. perhaps I remember that wrong, a bit under 4G should be ok.)
> >
> > > --cw
> >
> > /Matti Aarnio
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