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

[Tomoki Sekiyama <tomoki.sekiyama.qu@hitachi.com>]

Hi,
Thanks for your reply.

>>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.

Then, what do you think of the following idea?

(4) add `dirty_start_writeback_ratio' as percentage of memory,
    at which a generator of dirty pages itself starts writeback
    (that is, non-blocking ratio).

In this way, `dirty_ratio' is used as the blocking ratio, so we don't
need to modify the sysctl.conf etc. I think it's easier to understand
for administrators of systems, because the interface is similar as
`dirty_background_ratio' and`dirty_ratio.'

If this is OK, I'll repost the patch.

> 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.

To realize tuning with absolute values, I consider that we need to
modify handling of `dirty_background_ratio,' `dirty_ratio' and so on as
well as `dirty_start_writeback_ratio.' I think this should be done in
another patch if this feature is required.

Regards,
--
Tomoki Sekiyama
Hitachi, Ltd., Systems Development Laboratory