Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp uptime

[Wu Fengguang]

> > I'm only testing on a desktop with 2 drives. I use a simple test to
> > write 2gb to sda then 2gb to sdb while recording the threshold values.
> > On 2.6.39-rc3, after the 2nd write starts it take approx 90 seconds for
> > sda's threshold value to drop from its maximum to minimum and sdb's to
> > rise from min to max. So this seems much too slow for normal desktop
> > workloads. 
> 
> Yes.
> 
> > I haven't tested with this patch on 2.6.39-rc3 yet, but I'm just about
> > to set that up. 
> 
> It will sure help, but the problem is now the low-memory NAS servers..
> 
> Fortunately my patchset could make the dirty pages ramp up much more
> fast than the ramp up speed of the per-bdi threshold, and is also less
> sensitive to the fluctuations of per-bdi thresholds in JBOD setup.

Look at the attached graph. You cannot notice an obvious "rampup"
stage in the number of dirty pages (red line) at all :)

Thanks,
Fengguang

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