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    SubjectRe: [RFC][PATCH 0/3] Skip I/O merges when disabled
    On Thu, Apr 24 2008, Alan D. Brunelle wrote:
    > Jens Axboe wrote:
    > > On 24/04/2008, at 15.29, Andi Kleen <> wrote:
    > >
    > >> "Alan D. Brunelle" <> writes:
    > >>
    > >>> The block I/O + elevator + I/O scheduler code spends a lot of time
    > >>> trying to merge I/Os -- rightfully so under "normal" circumstances.
    > >>> However, if one were to know that the incoming I/O stream was /very/
    > >>> random in nature, the cycles are wasted. (This can be the case, for
    > >>> example, during OLTP-type runs.)
    > >>>
    > >>> This patch stream adds a per-request_queue tunable that (when set)
    > >>> disables merge attempts, thus freeing up a non-trivial amount of CPU
    > >>> cycles.
    > >>
    > >> It sounds interesting. But explicit tunables are always bad because
    > >> they will be only used by a elite few. Do you think it would be
    > >> possible instead to keep some statistics on how successfull merging is
    > >> and
    > >> when the success rate is very low disable it automatically for some
    > >> time until a time out?
    > >>
    > >> This way nearly everybody could get most of the benefit from this
    > >> change.
    > >
    > > Not a good idea IMHO, it's much better with an explicit setting. That
    > > way you don't introduce indeterministic behavior.
    > Another way to attack this would be to have a user level daemon "watch
    > things" -
    > o We could leave 'nomerges' alone: if someone set that, they "know"
    > what they are doing, and we just don't attempt merges. [This tunable
    > would really be for the "elite few" - those that no which devices are
    > used in which ways - people that administer Enterprise load environments
    > tend to need to know this.]
    > o The kernel already exports stats on merges, so the daemon could watch
    > those stats in comparison to the number of I/Os submitted. If it
    > determined that merge attempts were not being very successful, it could
    > turn off merges for a period of time. Later it could turn them back on,
    > watch for a while, and repeat.
    > Does this sound better/worthwhile?

    That's is true, you could toggle this from a user daemon if you wish. I
    still think it's a really bad idea, but at least then it's entirely up
    to the user. I'm not a big fan of such schemes, to say the least.

    Jens Axboe

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