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    SubjectRe: [Patch 1/4] Delay accounting: Initialization
    Parag Warudkar wrote:
    > On Nov 15, 2005, at 5:29 PM, Shailabh Nagar wrote:
    >>> Does this mean, whether or not the per task delay accounting is used,
    >>> we have a constant overhead of sizeof(spinlock_t) + 2*sizeof (uint32_t)
    >>> + 2* sizeof(uint64_t) bytes going into the struct task_struct?. Is it
    >>> possible/beneficial to use struct task_delay_info *delays instead and
    >>> allocate it if task wants to use the information?
    >> Doing so would have value in the case where the feature is configured
    >> but no one ever registers to listen for it.
    > Precisely. Such a feature will be used only occasionally I suppose.
    > I didn't read the code deeply but are any scheduling decisions altered
    > based on this data? If not, then it makes sense to not account unless required.
    > I think it should be possible to do it on demand, per process instead
    > of forcing the accounting on _all_ processes which cumulatively becomes a sizeable
    > o/h.
    > Per Process activation of this feature will add significant value IMHO.
    > (Of course, if that's possible in first place.)

    Per-task activation is useful/possible only for long-running tasks.
    If one is trying to gather stats for a user-defined grouping of tasks
    then it would involve too much overhead & inaccuracy to require monitoring
    to be turned on individually.

    >> The cost of doing this would be
    >> - adding more code to the fork path to allocate conditionally
    > Just an unlikely branch for normal code path - not a big deal.
    > Also I am thinking it could be handled outside of fork?

    only if per-task activation is done - thats probably what you meant ?

    >> - make the collecting of the delays conditional on a similar check
    > Weighing this against the actual accounting - I think it's a win.

    Hmmm..since there is locking involved in the stats collection, this is
    starting to make a lot of sense.

    >> - cache pollution from following an extra pointer in the pgflt/
    >> io_schedule paths
    >> I'm not sure is this really matters for these two code paths.
    > Possibly.
    >> Even if one does this, once the first listener registers, all future
    >> tasks
    >> (and even the current ones) will have to go ahead and allocate the
    >> structure
    >> and accounting of delays will have to switch to unconditional mode.
    >> This is
    >> because the delay data has cumulative value...future listeners will be
    >> interested in data collected earlier (as long as task is still
    >> running). And
    >> once the first listener registers, you can no longer be sure no one's
    >> interested
    >> in the future.
    > Is it possible to do it per process? Forcing it on all processes is
    > what I was trying to avoid given the feature's usage pattern.
    >> Another alternative is to let userland control the overhead of
    >> allocation and
    >> collection completely through a /proc/sys/kernel/delayacct variable.
    >> When its switched on, it triggers an allocation for all existing
    >> tasks in the
    >> system, turns on allocation in fork() for future tasks, and
    >> collection of the stats.
    >> When turned off, collection of stats stops as does allocation for
    >> future tasks
    >> (not worth going in and deallocating structs for existing tasks).
    >> Does this seem worth it ?
    > Definitely not unless we can do it per process and on demand.

    Per-task doesn't seem like a good idea. Neither does allowing dynamic
    switching off/on of the allocation of the delay struct.

    So how about this:

    Have /proc/sys/kernel/delayacct and a corresponding kernel boot parameter (for setting
    the switch early) which control just the collection of data. Allocation always happens.

    >> -- Shailabh
    > Cheers
    > Parag

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