Re: [PATCH 10/18] writeback: dirty position control - bdi reservearea

[Wu Fengguang]

> BTW, I also compared the IO-less patchset and the vanilla kernel's
> JBOD performance. Basically, the performance is lightly improved
> under large memory, and reduced a lot in small memory servers.
> 
>  vanillla IO-less  
> --------------------------------------------------------------------------------
[...]
>  26508063 17706200      -33.2%  JBOD-10HDD-thresh=100M/xfs-100dd-1M-16p-5895M-100M
>  23767810 23374918       -1.7%  JBOD-10HDD-thresh=100M/xfs-10dd-1M-16p-5895M-100M
>  28032891 20659278      -26.3%  JBOD-10HDD-thresh=100M/xfs-1dd-1M-16p-5895M-100M
>  26049973 22517497      -13.6%  JBOD-10HDD-thresh=100M/xfs-2dd-1M-16p-5895M-100M
> 
> There are still some itches in JBOD..

OK, in the dirty_bytes=100M case, I find that the bdi threshold _and_
writeout bandwidth may drop close to 0 in long periods. This change
may avoid one bdi being stuck:

        /*
         * bdi reserve area, safeguard against dirty pool underrun and disk idle
         *
         * It may push the desired control point of global dirty pages higher
         * than setpoint. It's not necessary in single-bdi case because a
         * minimal pool of @freerun dirty pages will already be guaranteed.
         */
-       x_intercept = min(write_bw, freerun);
+       x_intercept = min(write_bw + MIN_WRITEBACK_PAGES, freerun);
        if (bdi_dirty < x_intercept) {
                if (bdi_dirty > x_intercept / 8) {
                        pos_ratio *= x_intercept;
                        do_div(pos_ratio, bdi_dirty);
                } else
                        pos_ratio *= 8;
        }

Thanks,
Fengguang
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