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    SubjectRe: [patch] SLQB slab allocator (try 2)
    On Mon, 2009-01-26 at 12:22 -0500, Christoph Lameter wrote:
    > On Mon, 26 Jan 2009, Peter Zijlstra wrote:
    > > Then again, anything that does allocation is per definition not bounded
    > > and not something we can have on latency critical paths -- so on that
    > > respect its not interesting.
    > Well there is the problem in SLAB and SLQB that they *continue* to do
    > processing after an allocation. They defer queue cleaning. So your latency
    > critical paths are interrupted by the deferred queue processing.

    No they're not -- well, only if you let them that is, and then its your
    own fault.

    Remember, -rt is about being able to preempt pretty much everything. If
    the userspace task has a higher priority than the timer interrupt, the
    timer interrupt just gets to wait.

    Yes there is a very small hardirq window where the actual interrupt
    triggers, but all that that does is a wakeup and then its gone again.

    > SLAB has
    > the awful habit of gradually pushing objects out of its queued (tried to
    > approximate the loss of cpu cache hotness over time). So for awhile you
    > get hit every 2 seconds with some free operations to the page allocator on
    > each cpu. If you have a lot of cpus then this may become an ongoing
    > operation. The slab pages end up in the page allocator queues which is
    > then occasionally pushed back to the buddy lists. Another relatively high
    > spike there.

    Like Nick has been asking, can you give a solid test case that
    demonstrates this issue?

    I'm thinking getting git of those cross-bar queues hugely reduces that

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