Re: Annoying /proc/net/dev rollovers.

[Jeff Garzik <jgarzik@pobox.com>]

Mark J Roberts wrote:
> Chris Wedgwood:
>>>total_rx_bytes += rx_bytes;
>>
>>if lval is 64-bit, then this cannot be done reliably on all
>>architectures
> 
> 
> I'm not sure why. I realize that x86 can't do atomic 64-bit
> operations, but what I propose is to leave the 32-bit rx_bytes code
> the way it is, and just have some heuristic for updating the 64-bit
> value every so often, which can be done under a lock, so there would
> be no opportunity for races to corrupt the counter. (This is also an
> optimization since there needn't be any locks in the actual packet
> handling code.)


I was one of the ones who was interested in making the statistics 
64-bit, and adding locking to do it right.  The solution finally 
appeared, many months ago:

The counters don't need to be 64-bit, because it is trivially possible 
for userspace to track the statistics, and to simply use the difference 
between two samples as the increment used in calculating whatever 
numbers you wish -- 64-bit SNMP MIB statistics were what I was 
interested in.  Wrapping is trivially handled by standard unsigned int 
arithmetic, among other methods.

If you really want the raw data, then use ethtool's NIC-specific stats 
facility, to retrieve raw statistics directly from the NIC.  [this of 
course requires driver modifications, but they are easy on modern NICs]

	Jeff