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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
