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Subject[PATCH 0/3] ipc/sem.c: Optimization for reducing spinlock contention
Date
Hi,

I've cleaned up the patches I sent a week ago:
- they pass now checkpatch.pl
- some comments updated/added
- a bug with semctl(,,SETALL,) is fixed
- tested against LTP

Andrew: Could you add them the next -mm?

The following series of patches tries to fix the spinlock contention
reported by Chris Manson: His benchmark exposes problems of the current
code:

- In the worst case, the algorithm used by update_queue() is O(N^2).
Bulk wake-up calls can enter this worst case.
The patch series fix that.
Note that the benchmark app doesn't expose the problem, it just should
be fixed: Real world apps might do the wake-ups in another order
than perfect FIFO.

- The part of the code that runs within the semaphore array spinlock
is significantly larger than necessary.
The patch series fixes that. This change is responsible for the
main improvement.

- The cacheline with the spinlock is also used for a variable that
is read in the hot path (sem_base) and for a variable that is
unnecessarily written to multiple times (sem_otime).
The last step of the series cacheline-aligns the spinlock.

--
Manfred


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