Messages in this thread | | | Date | Fri, 1 May 2009 13:56:39 -0400 (EDT) | From | Steven Rostedt <> | Subject | Re: [PATCH 3/3] ring-buffer: make cpu buffer entries counter atomic |
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On Fri, 1 May 2009, Ingo Molnar wrote: > > > > Ah, maybe there's confusion in my explanation. When I talk about > > writers and readers, I'm talking about those writers into the ring > > buffer and readers from the ring buffer. But both writers and > > readers write to the entries counter. Readers subtract and writers > > add. But writers can also subtract on overruns. > > a solution for that would be to split it into two counts - for both > sides. Or to eliminate it if possible. We _really_ need to make the > ring-buffer _much_ cheaper than it is today.
I was thinking the same thing :-)
Actually we would have three counters. All incremental.
entries overruns read
The writer when adding an entry would increment entries
The writer when overwriting will increment overruns
The reader would increment read.
We could even make it 64 bits (or more) on all archs. Heck, make it two longs (128 bits for 64 bit archs). Since we only need to worry about interrupts, we only increment the second counter if we wrapped the first. I doubt a single interrupt (or many) could wrap the first counter. If it could, we have more to worry about than counters.
Thus the number of entries in the buffer would be:
(entries - overruns) - read
-- Steve
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