Messages in this thread | | | Date | Fri, 20 Mar 2009 20:09:12 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC][PATCH 3/3] perf_counter: new output ABI - part 1 |
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* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> Rework the output ABI > > use sys_read() only for instant data and provide mmap() output for > all async overflow data.
i like this.
> The first mmap() determines the size of the output buffer. The > mmap() size must be a PAGE_SIZE multiple of 1+pages, where pages > must be a power of 2 or 0. Further mmap()s of the same fd must > have the same size. Once all maps are gone, you can again mmap() > with a new size. > > In case of 0 extra pages there is no data output and the first > page only contains meta data. > > When there are data pages, a poll() event will be generated for > each full page of data. Furthermore, the output is circular. This > means that although 1 page is a valid configuration, its useless, > since we'll start overwriting it the instant we report a full > page.
i think it would still be nice to allow plain old-fashioned poll()+read() loops ... but the logistics of that seem difficult. mmap() seems to fit this better - and it's probably faster as well. (as we have to construct the kernel-space pages anyway, so mapping them isnt that big of an issue)
per-CPU-ness will be handled naturally via per-cpu counters.
Paul, can you see any hole/quirkiness in this scheme?
Ingo
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