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    SubjectRe: Spinlock performance on Athlon MP (2.4)
       From: Alan Cox <>
    Date: 30 Jul 2003 23:59:00 +0100

    On Mer, 2003-07-30 at 22:50, Scott L. Burson wrote:
    > First, and probably the reason you haven't heard more complaints about the
    > problem, its severity is evidently dependent on the size of main memory. At
    > 512MB it doesn't seem to be much of a problem (right, Mathieu?). At 2.5GB,
    > which is what I have, it can be quite serious. For instance, if I start two
    > `find' processes at the roots of different filesystems, the system can spend
    > (according to `top') 95% - 98% of its time in the kernel. It even gets
    > worse than that, but `top' stops updating -- in fact, the system can seem
    > completely frozen, but it does recover eventually. Stopping or killing one
    > of the `find' processes brings it back fairly quickly, though it can take a
    > while to accomplish that.

    Thats the well understood DMA bounce buffers problem.

    It's definitely not the bounce buffers problem. I installed the patch and
    it doesn't help (well, maybe it helps a little; it's hard to tell).

    However, I have pretty strong evidence that it's not the spinlock handoff
    time either. I wrote a small benchmark that starts two threads that do
    nothing but hand two spinlocks back and forth. The Athlon runs it an order
    of magnitude _faster_ than the P4 (5ns vs. 50ns, roughly, per handoff).

    I'm fairly certain that lock contention is involved somehow, though.
    Lockmeter reports that lock waiting is consuming about 35% of the CPU cycles
    when the problem is happening. This isn't the 90% - 95% number I expected
    -- the latter being the percentage of time spent in the kernel, as reported
    by `top' -- but it's high enough to wonder about, and it may be artificially
    low. Lockmeter has a spinlock that protects its data structures, and the
    profile says 36% of the time is being spent in the routine that acquires
    that lock. This suggests that lockmeter isn't counting that time.

    One oddity pointed up by lockmeter is that `pagemap_lru_lock' is held by
    `shrink_cache' some 85% of the time. This seems way too high, and I am
    looking into it.

    -- Scott
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