Messages in this thread | | | Date | Fri, 25 Jul 2003 17:44:20 -0700 | From | jw schultz <> | Subject | Re: Net device byte statistics |
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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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