Re: [PATCH] writeback: enabling-gate for light dirtied bdi

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On 12/05/2010 01:44 AM, Wu Fengguang wrote:
I noticed that my NFSROOT test system goes slow responding when there
is heavy dd to a local disk. Traces show that the NFSROOT's bdi_limit
is near 0 and many tasks in the system are repeatedly stuck in
balance_dirty_pages().

There are two related problems:

- light dirtiers at one device (more often than not the rootfs) get
   heavily impacted by heavy dirtiers on another independent device

- the light dirtied device does heavy throttling because bdi_limit=0,
   and the heavy throttling may in turn withhold its bdi_limit in 0 as
   it cannot dirty fast enough to grow up the bdi's proportional weight.

Fix it by introducing some "low pass" gate, which is a small (<=8MB)
value reserved by others and can be safely "stole" from the current
global dirty margin.  It does not need to be big to help the bdi gain
its initial weight.

Makes a lot of sense to me.

Acked-by: Rik van Riel <riel@xxxxxxxxxx>

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