Re: [PATCH 02/18] writeback: dirty position control

[Wu Fengguang]

On Wed, Sep 07, 2011 at 02:20:34AM +0800, Vivek Goyal wrote:
> On Sun, Sep 04, 2011 at 09:53:07AM +0800, Wu Fengguang wrote:
> 
> [..]
> > - in memory tight systems, (1) becomes strong enough to squeeze dirty
> >   pages inside the control scope
> > 
> > - in large memory systems where the "gravity" of (1) for pulling the
> >   dirty pages to setpoint is too weak, (2) can back (1) up and drive
> >   dirty pages to bdi_setpoint ~= setpoint reasonably fast.
> > 
> > Unfortunately in JBOD setups, the fluctuation range of bdi threshold
> > is related to memory size due to the interferences between disks.  In
> > this case, the bdi slope will be weighted sum of write_bw and bdi_thresh.
> 
> Can you please elaborate a little more that what changes in JBOD setup.
> 
> > 
> > Given equations
> > 
> >         span = x_intercept - bdi_setpoint
> >         k = df/dx = - 1 / span
> > 
> > and the extremum values
> > 
> >         span = bdi_thresh
> >         dx = bdi_thresh
> > 
> > we get
> > 
> >         df = - dx / span = - 1.0
> > 
> > That means, when bdi_dirty deviates bdi_thresh up, pos_ratio and hence
> > task ratelimit will fluctuate by -100%.
> 
> I am not sure I understand above calculation. I understood the part that
> for single bdi case, you want 12.5% varation of bdi_setpoint over a
> range of write_bw [SP-write_bw/2, SP+write_bw/2]. This requirement will
> lead to.
> 
> k = -1/8*write_bw
> 
> OR span = 8*write_bw, hence
> k= -1/span

That's right.

> Now I missed the part that what is different in case of JBOD setup and
> how do you come up with values for that setup so that slope of bdi
> setpoint is sharper.
> 
> IIUC, in case of single bdi case you want to use k=-1/(8*write_bw) and in
> case of JBOD you want to use k=-1/(bdi_thresh)?

Yeah.

> That means for single bdi case you want to trust bdi, write_bw but in
> case of JBOD you stop trusting that and just switch to bdi_thresh. Not
> sure what does it mean.

The main differences are,

1) in JBOD setup, bdi_thresh is fluctuating; in single bdi case,
   bdi_thresh is pretty stable. The fluctuating bdi_thresh means
   even if bdi_dirty is stable, dx=(bdi_dirty-bdi_setpoint) will be
   fluctuating a lot. And the dx range is no long bounded by the
   bdi write bandwidth, but proportional to bdi_thresh.

2) for single bdi case, bdi_dirty=nr_dirty is controlled by both
   the memory based global control line and the bandwidth based bdi
   control line. However for JBOD, we want to keep bdi_dirty reasonably
   close to bdi_setpoint, however the global control line is not going
   to help us directly. The bdi_thresh based slope can better serve
   this purpose than the write bandwidth.

Thanks,
Fengguang
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