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

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

Hi Kamezawa-san,

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.)
<snip>
>>> >>> 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.

> > What nr_request means ?

nr_requests is a parameter that means upper limit of the length of I/O
(read- and write-)requests queue of the device, which is configurable
from /sys/block/<device>/queue/nr_requests. A process, which perform I/O
when there are more than nr_requests requests in the queue, will be blocked.

> > But Ok, maybe I'm not understanding. What I want to ask you is do
> > per-device-write-throttling rather than adding a new parameter.

In the patchset, per-device-write-throttling is done by the behavior
of the write-requests queue described above.

When the queue of the disk becomes full while writeback of Dirty in
writeback_inodes(), heavy writes to the disk will be blocked.
In contrast, if it's so occasional that the queue doesn't become full,
writes will not be blocked.


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