Messages in this thread | | | Date | Thu, 24 Apr 2008 17:05:34 +0200 | From | Jens Axboe <> | Subject | Re: [RFC][PATCH 0/3] Skip I/O merges when disabled |
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On Thu, Apr 24 2008, Alan D. Brunelle wrote: > Jens Axboe wrote: > > On 24/04/2008, at 15.29, Andi Kleen <andi@firstfloor.org> wrote: > > > >> "Alan D. Brunelle" <Alan.Brunelle@hp.com> 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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