Re: [PATCH 3/3] writeback: add dirty_ratio_time per bdi variable (NFS write performance)

[Namjae Jeon <linkinjeon-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

2012/8/19, Fengguang Wu <fengguang.wu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>:
> On Sat, Aug 18, 2012 at 05:50:02AM -0400, Namjae Jeon wrote:
>> From: Namjae Jeon <namjae.jeon-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>
>>
>> This patch is based on suggestion by Wu Fengguang:
>> https://lkml.org/lkml/2011/8/19/19
>>
>> kernel has mechanism to do writeback as per dirty_ratio and
>> dirty_background
>> ratio. It also maintains per task dirty rate limit to keep balance of
>> dirty pages at any given instance by doing bdi bandwidth estimation.
>>
>> Kernel also has max_ratio/min_ratio tunables to specify percentage of
>> writecache
>> to control per bdi dirty limits and task throtelling.
>>
>> However, there might be a usecase where user wants a writeback tuning
>> parameter to flush dirty data at desired/tuned time interval.
>>
>> dirty_background_time provides an interface where user can tune
>> background
>> writeback start time using /sys/block/sda/bdi/dirty_background_time
>>
>> dirty_background_time is used alongwith average bdi write bandwidth
>> estimation
>> to start background writeback.
>
> Here lies my major concern about dirty_background_time: the write
> bandwidth estimation is an _estimation_ and will sure become wildly
> wrong in some cases. So the dirty_background_time implementation based
> on it will not always work to the user expectations.
>
> One important case is, some users (eg. Dave Chinner) explicitly take
> advantage of the existing behavior to quickly create & delete a big
> 1GB temp file without worrying about triggering unnecessary IOs.
>
Hi. Wu.
Okay, I have a question.

If making dirty_writeback_interval per bdi to tune short interval
instead of background_time, We can get similar performance
improvement.
/sys/block/<device>/bdi/dirty_writeback_interval
/sys/block/<device>/bdi/dirty_expire_interval

NFS write performance improvement is just one usecase.

If we can set interval/time per bdi,  other usecases will be created
by applying.

How do you think ?

>The numbers are impressive! FYI, I tried another NFS specific approach
>to avoid big NFS COMMITs, which achieved similar performance gains:

>nfs: writeback pages wait queue
>https://lkml.org/lkml/2011/10/20/235

Thanks.
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