• Nicholas Piggin's avatar
    powerpc/64s: Implement queued spinlocks and rwlocks · aa65ff6b
    Nicholas Piggin authored
    These have shown significantly improved performance and fairness when
    spinlock contention is moderate to high on very large systems.
    
    With this series including subsequent patches, on a 16 socket 1536
    thread POWER9, a stress test such as same-file open/close from all
    CPUs gets big speedups, 11620op/s aggregate with simple spinlocks vs
    384158op/s (33x faster), where the difference in throughput between
    the fastest and slowest thread goes from 7x to 1.4x.
    
    Thanks to the fast path being identical in terms of atomics and
    barriers (after a subsequent optimisation patch), single threaded
    performance is not changed (no measurable difference).
    
    On smaller systems, performance and fairness seems to be generally
    improved. Using dbench on tmpfs as a test (that starts to run into
    kernel spinlock contention), a 2-socket OpenPOWER POWER9 system was
    tested with bare metal and KVM guest configurations. Results can be
    found here:
    
    https://github.com/linuxppc/issues/issues/305#issuecomment-663487453
    
    Observations are:
    
    - Queued spinlocks are equal when contention is insignificant, as
      expected and as measured with microbenchmarks.
    
    - When there is contention, on bare metal queued spinlocks have better
      throughput and max latency at all points.
    
    - When virtualised, queued spinlocks are slightly worse approaching
      peak throughput, but significantly better throughput and max latency
      at all points beyond peak, until queued spinlock maximum latency
      rises when clients are 2x vCPUs.
    
    The regressions haven't been analysed very well yet, there are a lot
    of things that can be tuned, particularly the paravirtualised locking,
    but the numbers already look like a good net win even on relatively
    small systems.
    Signed-off-by: default avatarNicholas Piggin <npiggin@gmail.com>
    Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Acked-by: default avatarWaiman Long <longman@redhat.com>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20200724131423.1362108-4-npiggin@gmail.com
    aa65ff6b
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