Re: [PATCH 0/3] VM throttling: avoid blocking occasional writers

[Bill Davidsen <davidsen@tmr.com>]

Tomoki Sekiyama wrote:
> Hi,
>
> Thanks for your comments.
> I'm sorry for my late reply.
>
> Bill Davidsen wrote:
> > Andrew Morton wrote:
>
> >> - I wonder if dirty_limit_ratio is the best name we could choose.
> >> vm_dirty_blocking_ratio, perhaps?  Dunno.
> >>
> > I don't like it, but I dislike it less than "dirty_limit_ratio" I 
> guess.
> > It would probably break things to change it now, including my
> > sysctl.conf on a number of systems  :-(
>
> I'm wondering which interface is preferred...
>
> 1) Just rename "dirty_limit_ratio" to "dirty_blocking_ratio."
>    Those who had been changing dirty_ratio should additionally modify
>    dirty_blocking_ratio in order to determine the upper limit of dirty 
> pages.
>
> 2) Change "dirty_ratio" to a vector, consists of 2 values;
>    {blocking ratio, writeback starting ratio}.
>    For example, to change the both values:
>      # echo 40 35 > /proc/sys/vm/dirty_ratio
>    And to change only the first one:
>      # echo 20 > /proc/sys/vm/dirty_ratio
>    In the latter way the writeback starting ratio is regarded as the 
> same as the
>    blocking ratio if the writeback starting ratio is smaller. And 
> then, the kernel behaves
>    similarly as the current kernel.
>
> 3) Use "dirty_ratio" as the blocking ratio. And add
>    "start_writeback_ratio", and start writeback at
>    start_writeback_ratio(default:90) * dirty_ratio / 100 [%].
>    In this way, specifying blocking ratio can be done in the same way as
>    current kernel, but high/low watermark algorithm is enabled.
I like 3 better, it should make tuning behavior more precise. You can 
make an argument for absolute values for writeback, if my disk will only 
write 70MB/s I may only want 203 sec of pending writes, regardless of 
available memory.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979