Jens Axboe wrote:
ATM we have one timer for both data and super-block synchronization.
With per-bdi write-back we have:
1. one timer for super blocks
2. many per-bdi timers for data (schedule_timeout() is essentially
using timers).
That is correct. Note that these exit when they have been idle for a
while, for embedded and such you could make it more aggressive by
exiting quicker. The sync_supers should be directly fixable by your
sb_dirty() stuff.
So I don't think it's a huge change from what we currently have.
This is not nice, because each timer is an additional source of
power-savings killers. I mean, it is more power management (PM)
friendly to have less timers and disturb CPU less, make CPU wake
up from retention less frequently.
I do not challange the per-bdi idea at all, but is it possible to
think about a more PM-friendly desing and have one source of
periodic write-back, not many. I mean, could there be one timer
which periodically syncs supers and wakes up the BDI write-back
tasks?
You could replace the schedule_timeout() by a schedule(), and instead
have a single timer running that would scan the bdi_list and issue the
kupdated() timed writeback that is the reason it uses schedule_timeout()
now. Explicitly issued work will manually wake up the per-bdi thread(s).
That single timer could easily handle waking up bdi_sync_supers() as
well.
Right. May be the way you decomposed stuff will actually make it easier
to to optimize periodic write-back and teach it not to wake up if
there is no dirt.
And we could as well use rang hrtimers to optimize events grouping.
I'll keep looking at this.
--
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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