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

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

Hi Nikita,

thanks for your comments.

Nikita Danilov wrote:
>> While Dirty+Writeback pages get more than 40% of memory, process-B is
>> blocked in balance_dirty_pages() until writeback of some (`write_chunk',
>> typically = 1536) dirty pages on disk-b is started.
>
> May be the simpler solution is to use separate variables to control
> ratelimit and write chunk?

No, I think it's difficult to throttle total Dirty+Writeback only with
write_chunk, because write_chunk just affects Dirty and Writeback of
each device (in this case, throttling is done in write-requests queue of
the each backing device, as I said in another mail).

Throttling of the total Dirty+Writeback should be also done in VM itself,
and to control that, I added `dirty_limit_ratio.'


> writeback_set_ratelimit() adjusts ratelimit_pages to avoid too frequent
> calls to balance_dirty_pages(), but once we are inside of
> writeback_inodes(), there is no need to write especially many pages in
> one go: overhead of any additional looping is negligible, when compared
> with the cost of writing.
>
> Speaking of which, now that expensive get_writeback_state() is gone from
> page-writeback.c why do we need adjustable ratelimiting at all? It looks
> like writeback_set_ratelimit() can be dropped, and fixed ratelimit used
> instead.

As far as I can see, adjustable ratelimiting is the actual cause of the
long wait on writing to disk with light load.
I think removing adjustable ratelimiting should be done in another patch...


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