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

[Wu Fengguang <fengguang.wu@intel.com>]

On Sun, Sep 18, 2011 at 10:17:05PM +0800, Wu Fengguang wrote:
> On Mon, Sep 12, 2011 at 06:19:38PM +0800, Peter Zijlstra wrote:
> > On Wed, 2011-09-07 at 20:31 +0800, Wu Fengguang wrote:
> > > > > +   x_intercept = min(write_bw, freerun);
> > > > > +   if (bdi_dirty < x_intercept) {
> > > > 
> > > > So the point of the freerun point is that we never throttle before it,
> > > > so basically all the below shouldn't be needed at all, right? 
> > > 
> > > Yes!
> > > 
> > > > > +           if (bdi_dirty > x_intercept / 8) {
> > > > > +                   pos_ratio *= x_intercept;
> > > > > +                   do_div(pos_ratio, bdi_dirty);
> > > > > +           } else
> > > > > +                   pos_ratio *= 8;
> > > > > +   }
> > > > > +
> > > > >     return pos_ratio;
> > > > >  }
> > 
> > Does that mean we can remove this whole block?
> 
> Right, if the bdi freerun concept is proved to work fine.
> 
> Unfortunately I find it mostly yields lower performance than bdi
> reserve area. Patch is attached. If you would like me try other
> patches, I can easily kick off new tests and redo the comparison.
> 
> Here is the nr_written numbers over various JBOD test cases,
> the larger, the better:
> 
> bdi-reserve     bdi-freerun    diff    case
> ---------------------------------------------------------------------------------------
> 38375271        31553807      -17.8%	JBOD-10HDD-6G/xfs-100dd-1M-16p-5895M-20
> 30478879        28631491       -6.1%	JBOD-10HDD-6G/xfs-10dd-1M-16p-5895M-20
> 29735407        28871956       -2.9%	JBOD-10HDD-6G/xfs-1dd-1M-16p-5895M-20
> 30850350        28344165       -8.1%	JBOD-10HDD-6G/xfs-2dd-1M-16p-5895M-20
> 17706200        16174684       -8.6%	JBOD-10HDD-thresh=100M/xfs-100dd-1M-16p-5895M-100M
> 23374918        14376942      -38.5%	JBOD-10HDD-thresh=100M/xfs-10dd-1M-16p-5895M-100M
> 20659278        19640375       -4.9%	JBOD-10HDD-thresh=100M/xfs-1dd-1M-16p-5895M-100M
> 22517497        14552321      -35.4%	JBOD-10HDD-thresh=100M/xfs-2dd-1M-16p-5895M-100M
> 68287850        61078553      -10.6%	JBOD-10HDD-thresh=2G/xfs-100dd-1M-16p-5895M-2048M
> 33835247        32018425       -5.4%	JBOD-10HDD-thresh=2G/xfs-10dd-1M-16p-5895M-2048M
> 30187817        29942083       -0.8%	JBOD-10HDD-thresh=2G/xfs-1dd-1M-16p-5895M-2048M
> 30563144        30204022       -1.2%	JBOD-10HDD-thresh=2G/xfs-2dd-1M-16p-5895M-2048M
> 34476862        34645398       +0.5%	JBOD-10HDD-thresh=4G/xfs-10dd-1M-16p-5895M-4096M
> 30326479        30097263       -0.8%	JBOD-10HDD-thresh=4G/xfs-1dd-1M-16p-5895M-4096M
> 30446767        30339683       -0.4%	JBOD-10HDD-thresh=4G/xfs-2dd-1M-16p-5895M-4096M
> 40793956        45936678      +12.6%	JBOD-10HDD-thresh=800M/xfs-100dd-1M-16p-5895M-800M
> 27481305        24867282       -9.5%	JBOD-10HDD-thresh=800M/xfs-10dd-1M-16p-5895M-800M
> 25651257        22507406      -12.3%	JBOD-10HDD-thresh=800M/xfs-1dd-1M-16p-5895M-800M
> 19849350        21298787       +7.3%	JBOD-10HDD-thresh=800M/xfs-2dd-1M-16p-5895M-800M

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  
--------------------------------------------------------------------------------
 31189025 34476862      +10.5%  JBOD-10HDD-thresh=4G/xfs-10dd-1M-16p-5895M-4096M
 30441974 30326479       -0.4%  JBOD-10HDD-thresh=4G/xfs-1dd-1M-16p-5895M-4096M
 30484578 30446767       -0.1%  JBOD-10HDD-thresh=4G/xfs-2dd-1M-16p-5895M-4096M

 68532421 68287850       -0.4%  JBOD-10HDD-thresh=2G/xfs-100dd-1M-16p-5895M-2048M
 31606793 33835247       +7.1%  JBOD-10HDD-thresh=2G/xfs-10dd-1M-16p-5895M-2048M
 30404955 30187817       -0.7%  JBOD-10HDD-thresh=2G/xfs-1dd-1M-16p-5895M-2048M
 30425591 30563144       +0.5%  JBOD-10HDD-thresh=2G/xfs-2dd-1M-16p-5895M-2048M

 40451069 38375271       -5.1%  JBOD-10HDD-6G/xfs-100dd-1M-16p-5895M-20
 30903629 30478879       -1.4%  JBOD-10HDD-6G/xfs-10dd-1M-16p-5895M-20
 30113560 29735407       -1.3%  JBOD-10HDD-6G/xfs-1dd-1M-16p-5895M-20
 30181418 30850350       +2.2%  JBOD-10HDD-6G/xfs-2dd-1M-16p-5895M-20

 46067335 40793956      -11.4%  JBOD-10HDD-thresh=800M/xfs-100dd-1M-16p-5895M-800M
 30425063 27481305       -9.7%  JBOD-10HDD-thresh=800M/xfs-10dd-1M-16p-5895M-800M
 28437929 25651257       -9.8%  JBOD-10HDD-thresh=800M/xfs-1dd-1M-16p-5895M-800M
 29409406 19849350      -32.5%  JBOD-10HDD-thresh=800M/xfs-2dd-1M-16p-5895M-800M

 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..

Thanks,
Fengguang