Messages in this thread | | | Date | Thu, 24 Apr 2008 10:13:05 -0400 | From | "Alan D. Brunelle" <> | Subject | Re: [RFC][PATCH 0/3] Skip I/O merges when disabled |
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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?
Alan
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