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    SubjectRe: /proc reliability & performance
    On Thu, Oct 16, 2003 at 10:07:18PM -0400, Albert Cahalan wrote:
    > Tie directory readers to a task_struct (or to
    > some of the PID tracking structs), so that
    > a directory reader is on a list. When a task
    > exits, move the list of directory readers on
    > to a neighboring task.
    > That is O(1) on task exit, and generally O(n)
    > for the whole /proc or /proc/42/task read.
    > It's O(1) per step of the read, excepting
    > where multiple directory readers wind up at
    > the same location.
    > Another benefit is that it is reliable as
    > long as tasks don't move around on the lists.
    > Each task will appear at most once, and will
    > appear exactly once if it doesn't start or
    > exit during the directory scan.

    Several other things have been tried.
    (a) something mingo wrote I forgot the nature of
    (b) a thing manfred wrote that recovers positions in hashtable
    collision chains by sorting them, with O(chain length)
    (c) a thing I wrote that turns the tasklist and pid_chains into
    rbtrees and uses the last-seen pid to seek in O(lg(n))
    time, and uses a routine to seek and fill buffers as a
    drop-in replacement for get_tgid_list()/get_tid_list().

    I have a current implementation of (c), as well as a patch to
    restore 2.4 semantics to proc_pid_statm() in O(1) time.

    -- wli
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