Re: Net device byte statistics

[jw schultz <jw@pegasys.ws>]

On Fri, Jul 25, 2003 at 05:08:46PM -0700, Ben Greear wrote:
> Jeff Sipek wrote:
> >-----BEGIN PGP SIGNED MESSAGE-----
> >Hash: SHA1
> >
> >On Friday 25 July 2003 17:55, jw schultz wrote:
> >
> 
> >>My thought would be to use 96bits for each counter.  In-kernel
> >>code would run periodically doing something like this:
> >>
> >>	curval = counter.in_kernel;
> >>			/* get it in a register for atomicity */
> >>	if (counter.user_low < curval)
> >>		++counter.user_high;
> >>	counter.user_low = curval;
> 
> What about every 30 seconds or so, detect wraps, and bump the 'high' counter
> if it wraps.  (Check more often if you can wrap more than once in 30 secs).

Yes, how often the component needs run will depend on the
fastest counter.

> Then, upon read by user-space (or whatever needs 64-bit counters):
> 
> 1) check wrap
> 2) grab low bits and OR them with the high bits.
> 3) check wrap again.  If wrap happened, try again.  Assumption is it could 
> never wrap
>    more than once during the time you are checking.

If you need to have userspace get instantaneous values it
would be more efficient to have userspace do the
update_64bit_counter code for just its counter than to have
multiple wrap checks.

> I think this could give us very low overhead, and extremely precise 64-bit
> reads.  And, I think it would not need locks in the fast path..but I could
> also be missing something :)

Per-cpu counters.  If this is done a variant of this for
per-cpu counters would be helpful.  Per-cpu counters have
the advantage of reducing cache-line bouncing.  I don't
think per-cpu counters should be used as a band-aid
(elasto-plast) for counter wrapping.  Besides, how many
12Ghz 4-way hypertheaded (shared cache) CPUs do you need?
And if you only have one should you have per-cpu counters?
I don't think so.  

-- 
________________________________________________________________
	J.W. Schultz            Pegasystems Technologies
	email address:		jw@pegasys.ws

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