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    SubjectRe: andrea buffer code (2.2.9-C.gz)

    On Wed, 19 May 1999, Andrea Arcangeli wrote:

    > The point is that there we don't need a global spinlock but there we can
    > scale far more finegriend on per-page basis. If our approch is to scale
    > well as possible in SMP without bother to waste some more kbyte of memory
    > the spinlock it's required. [...]

    this is a mistake some other OSs did in the early SMP stage, _too_
    finegrained locking increases the size of kernel objects, increases cache
    footprint, sometimes increases the number of lock operations and thus
    slows things down.

    > [...] You know: to scale better you need to waste
    > more memory :-).

    Not necesserily, sometimes we need more locks, sometimes we need less. The
    point is to have Linux perform better. Putting a spinlock into every
    'struct page' wastes some memory (128k RAM on a 128M box, not much, but
    not small either), so this issue is not as easy to judge as you appear to
    think. Eg. i have threaded the SysV IPC code recently, and actually ended
    up _removing_ one particular type of spinlock because it only decreased
    measured performance.

    > >The spinlock is only acquired for a few lines,
    > >perhaps one global spinlock would save memory.
    > Yes but I am not worried, and being more finegrined in SMP is more fun :-).

    you might not be worried but others are ... Manfred has asked good
    questions: is the particular lock you have 'expanded' into per-object
    locks contended in tests, have you measured things. OS development is
    never 'mathematically clean'.

    -- mingo

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