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    SubjectRe: [patch 02/20] make the inode i_mmap_lock a reader/writer lock
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    On Wednesday 19 December 2007 08:15, Rik van Riel wrote:
    > I have seen soft cpu lockups in page_referenced_file() due to
    > contention on i_mmap_lock() for different pages. Making the
    > i_mmap_lock a reader/writer lock should increase parallelism
    > in vmscan for file back pages mapped into many address spaces.
    >
    > Read lock the i_mmap_lock for all usage except:
    >
    > 1) mmap/munmap: linking vma into i_mmap prio_tree or removing
    > 2) unmap_mapping_range: protecting vm_truncate_count
    >
    > rmap: try_to_unmap_file() required new cond_resched_rwlock().
    > To reduce code duplication, I recast cond_resched_lock() as a
    > [static inline] wrapper around reworked cond_sched_lock() =>
    > __cond_resched_lock(void *lock, int type).
    > New cond_resched_rwlock() implemented as another wrapper.

    Reader/writer locks really suck in terms of fairness and starvation,
    especially when the read-side is common and frequent. (also, single
    threaded performance of the read-side is worse).

    I know Lee saw some big latencies on the anon_vma list lock when
    running (IIRC) a large benchmark... but are there more realistic
    situations where this is a problem?


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