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    Subjectwhat is the -RT tree

    * Andi Kleen <> wrote:

    > > > > Yes, as Ingo stated many times, addition cond_resched() to
    > > > > might_sleep() does achieve the "usable" latencies -- and
    > > > > obviously that's hacky.
    > > >
    > > > But it's the only way to get practial(1) low latency benefit to
    > > > everybody [...]
    > > > (1) = not necessarily provable, but good enough at least for jack
    > > >
    > >
    > > FYI, to get good latencies for jack you currently need the -RT tree and
    > > CONFIG_PREEMPT. (see Lee Revell's and Rui Nuno Capela's extensive tests)
    > Yeah, but you did a lot of (often unrelated to rt preempt) latency
    > fixes in RT that are not yet merged into mainline. When they are all
    > merged things might be very different. And then there can be probably
    > more fixes.

    your argument above == cond_resched() in might_sleep() [ == VP ] is the
    only way to get practical (e.g. jack) latencies.

    my argument == i do agree that -VP is a step forward from PREEMPT_NONE
    (i'd not have written and released it otherwise), but is
    by no means enough for jack. You need at least the -RT
    tree + CONFIG_PREEMPT to achieve good jack latencies.

    in that sense your further argument that the -RT tree has more latency
    related fixes has no relevance to this point: VP by itself is _not
    enough_, having more latency fixes in the -RT tree (which mostly improve
    CONFIG_PREEMPT and PREEMPT_RT) does not make VP any better of a solution
    for jack's purposes.

    [ and yes, those other latency fixes are not necessarily directly
    related to the PREEMPT_RT feature, because the -RT tree is a
    collection of latency related fixes and features, of which PREEMPT_RT
    is the biggest, but not the only one. ]

    so my main point still remains: it's wishful thinking to expect the
    'standard' ( < CONFIG_PREEMPT) Linux kernel's latencies to improve well
    enough for jack.

    perhaps there's some misunderstanding wrt. what the -RT tree is. The -RT
    tree is a collection of latency related patches and features: it
    introduces the VP and PREEMPT_RT features, and it also improves all
    preemption models (including CONFIG_PREEMPT). Furthermore, it includes
    (in-kernel) features to measure and debug latencies. It's called -RT
    because PREEMPT_RT is undoubtedly the 'crown jewel' feature, but that
    does not mean it's the only goal of the patchset.

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      Last update: 2005-05-30 14:16    [W:0.033 / U:0.064 seconds]
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