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From: Robert Watson <rwatson@FreeBSD.org>
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To: Poul-Henning Kamp <phk@phk.freebsd.dk>
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Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org,
	src-committers@FreeBSD.org, Luigi Rizzo <rizzo@iet.unipi.it>
Subject: Re: per-cpu counters (Re: svn commit: r190967 - head/sys/netinet) 
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On Sun, 12 Apr 2009, Poul-Henning Kamp wrote:

> In message <alpine.BSF.2.00.0904121525540.19879@fledge.watson.org>, Robert 
> Wats on writes:
>
>> I have a project along these lines in progress, and will post the proposal 
>> to arch@ once I've finished prototyping it.  In particular, it provides 
>> common implementations of "reset" and "report" in order to expose a single 
>> userspace version of the structure via sysctl.
>
> Please don't export them via sysctl.
>
> Export them via mmap(2) like we already do with the disk I/O statistics.
>
> That way monitoring the counters becomes a no-syscall operation.

The sysctls already exist, and are used by both built-in and third-party 
monitoring tools.  Given the 8.0 timeline and the list of other things to get 
done before 8.0, I'm happy to solve the per-CPU counter problem as it is 
necessary to do so, but I don't have time to solve the mmap'd per-CPU counter 
problem, as it's not necessary to do so.  In the current design the per-CPU 
counter framework allocates the storage for the counters, so it should be easy 
to add that feature later.

Robert N M Watson
Computer Laboratory
University of Cambridge