Re: [RFC][PATCH 0/3] Skip I/O merges when disabled

["Alan D. Brunelle" <Alan.Brunelle@hp.com>]

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