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

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

Hi Kamezawa-san,

thanks for your reply.

KAMEZAWA Hiroyuki wrote:
> Interesting, but how about adjust this parameter like below instead of
> adding new control knob ?(this kind of knob is not easy to use.)
>
> ==
>                 struct writeback_control wbc = {
>                         .bdi            = bdi,
>                         .sync_mode      = WB_SYNC_NONE,
>                         .older_than_this = NULL,
>                         .nr_to_write    = 0,
>                         .range_cyclic   = 1,
>                 };
> <snip>
>                 if (nr_reclaimable) {
> 			/* Just do what I can do */
> 			dirty_pages_on_device = count_dirty_pages_on_device_limited(bdi, writechunk);
> 			wbc.nr_to_write = dirty_pages_on_device.
> 			writeback_inodes(&wbc);
>
> ==
>
> count_dirty_pages_on_device_limited(bdi, writechunk) above returns
> dirty pages on bdi. if # of dirty_pages on bdi is larger than writechunk,
> just returns writechunk.


I think that way is not enough to control the total amount of
Dirty+Writeback.

In that way, while writeback_inodes() scans for dirty pages and writes
them back, the caller will be blocked only if the length of the write-
requests queue is longer than nr_requests. If so, Writeback may consume
tens MB memory for each queue, because nr_requests is 128 and the
maximum size of a request is 512KB. If you have several devices, it can
consume more than hundred MB memory.

I concerned about that, so I introduced dirty_limit_ratio to limit the
total amount of Dirty+Writeback pages.


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