Re: [PATCH 7/8] writeback: sync old inodes first in backgroundwriteback

[Wu Fengguang]

> For the case of of a heavy dirtier (dd) and concurrent light dirtiers
> (some random processes), the light dirtiers won't be easily throttled.
> task_dirty_limit() handles that case well. It will give light dirtiers
> higher threshold than heavy dirtiers so that only the latter will be
> dirty throttled.

The caveat is, the real dirty throttling threshold is not exactly the
value specified by vm.dirty_ratio or vm.dirty_bytes. Instead it's some
value slightly lower than it. That real value differs for each process,
which is a nice trick to throttle heavy dirtiers first. If I remember
it right, that's invented by Peter and Andrew.

Thanks,
Fengguang
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