Re: [PATCH 2/5] writeback: dirty position control

[Wu Fengguang]

Hi Jan,

> > > >   What if x_intercept >  bdi_thresh? Since 8*bdi->avg_write_bandwidth is
> > > > easily 500 MB, that happens quite often I imagine?
> > > 
> > > That's fine because I no longer target "bdi_thresh" as some limiting
> > > factor as the global "thresh". Due to it being unstable in small
> > > memory JBOD systems, which is the big and unique problem in JBOD.
> >   I see. Given the control mechanism below, I think we can try this idea
> > and see whether it makes problems in practice or not. But the fact that
> > bdi_thresh is no longer treated as limit should be noted in a changelog -
> > probably of the last patch (although that is already too long for my taste
> > so I'll look into how we could make it shorter so that average developer
> > has enough patience to read it ;).
> 
> Good point. I'll make it a comment in the last patch.

Just added this comment:

+               /*
+                * bdi_thresh is not treated as some limiting factor as
+                * dirty_thresh, due to reasons
+                * - in JBOD setup, bdi_thresh can fluctuate a lot
+                * - in a system with HDD and USB key, the USB key may somehow
+                *   go into state (bdi_dirty >> bdi_thresh) either because
+                *   bdi_dirty starts high, or because bdi_thresh drops low.
+                *   In this case we don't want to hard throttle the USB key
+                *   dirtiers for 100 seconds until bdi_dirty drops under
+                *   bdi_thresh. Instead the auxiliary bdi control line in
+                *   bdi_position_ratio() will let the dirtier task progress
+                *   at some rate <= (write_bw / 2) for bringing down bdi_dirty.
+                */
                bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);

Thanks,
Fengguang
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