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    SubjectRe: [PATCH 2/3] High-res-timers part 2 (x86 platform code) take 5.1
    "Eric W. Biederman" wrote:
    >
    > george anzinger <george@mvista.com> writes:
    >
    > > Linus Torvalds wrote:
    > > >
    > > > On Wed, 9 Oct 2002, george anzinger wrote:
    > > > >
    > > > > This patch, in conjunction with the "core" high-res-timers
    > > > > patch implements high resolution timers on the i386
    > > > > platforms.
    > > >
    > > > I really don't get the notion of partial ticks, and quite frankly, this
    > > > isn't going into my tree until some major distribution kicks me in the
    > > > head and explains to me why the hell we have partial ticks instead of just
    > > > making the ticks shorter.
    > > >
    > > Well, the notion is to provide timers that have resolution
    > > down into the micro seconds. Since this take a bit more
    > > overhead, we just set up an interrupt on an as needed
    > > basis. This is why we define both a high res and a low res
    > > clock. Timers on the low res clock will always use the 1/HZ
    > > tick to drive them and thus do not introduce any additional
    > > overhead. If this is all that is needed the configure
    > > option can be left off and only these timers will be
    > > available.
    > >
    > > On the other hand, if a user requires better resolution,
    > > s/he just turns on the high-res option and incures the
    > > overhead only when it is used and then only at timer expire
    > > time. Note that the only way to access a high-res timer is
    > > via the POSIX clocks and timers API. They are not available
    > > to select or any other system call.
    > >
    > > Making ticks shorter causes extra overhead ALL the time,
    > > even when it is not needed. Higher resolution is not free
    > > in any case, but it is much closer to free with this patch
    > > than by increasing HZ (which, of course, can still be
    > > done). Overhead wise and resolution wise, for timers, we
    > > would be better off with a 1/HZ tick and the "on demand"
    > > high-res interrupts this patch introduces.
    >
    > ??? The issue of ticks is separate from the issue of how often
    > timer interrupts fire. Ticks just becomes the maximum resolution
    > you can support/express.
    >
    > If it makes sense to have two maximum tick resolutions. The normal
    > application maximum tick rate and the special task maximum tick
    > rate it is probably worth making this only available as a capability
    > or an rlimit.
    >
    I could support a notion that to use the high-res clock for
    a timer the user would need a particular capability. After
    all we do the same for the real time priority.

    Does this get us any closer to acceptance in 2.5?
    --
    George Anzinger george@mvista.com
    High-res-timers:
    http://sourceforge.net/projects/high-res-timers/
    Preemption patch:
    http://www.kernel.org/pub/linux/kernel/people/rml
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