linux-kernel - Re: [PATCH 10/18] writeback: dirty position control

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Date:	Sun, 18 Sep 2011 22:47:51 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/18] writeback: dirty position control - bdi reserve
 area

> 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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