4.8 ffs_dirpref problem

[Don Lewis]

On 31 Oct, To: kmarx at vicor.com wrote:
> On 31 Oct, Ken Marx wrote:

> You may get better results if you change the minbfree calculation from
> (avgbfree - avgbfree/4) to just (avgbfree).
> 
> I'm somewhat tempted to change the calculation to:
> 	min(avgbfree, max(1, (avgbfree - avgbfree/4), (dirsize/fs->fs_bsize)))
> where the last term works out to 4500 with your tunefs parameters.

I tried a variation of this on my -CURRENT box and it benchmarked
consistently worse.  I've got a "spare' 10 GB partition which first
copied my /usr/ports/packages to, and then filled by repeatedly tarring
my /usr/ports tree over to it.  The partition was 100% full, including
the reserve space, after four iterations.

With minbfree set to max((avgbfree - avgbfree/4), 1) here are two
iterations (the fifth line of timing data is for the 'rm -rf' command):

     1310.47 real         5.48 user       141.90 sys
     1336.78 real         5.62 user       152.27 sys
     1368.84 real         6.02 user       151.75 sys
     1359.70 real         5.55 user       154.01 sys
      423.44 real         2.25 user       107.26 sys

     1300.56 real         5.65 user       148.82 sys
     1372.20 real         5.79 user       152.25 sys
     1359.01 real         6.03 user       152.63 sys
     1380.90 real         5.31 user       153.71 sys
      437.22 real         2.20 user       105.61 sys

With minbfree set to
 max(min(max(avgbfree - avgbfree / 4, dirsize / fs->fs_bsize),
         avgbfree), 1)
I get the following:

     1314.61 real         5.66 user       175.43 sys
     1350.40 real         6.12 user       179.15 sys
     1386.86 real         6.32 user       179.12 sys
     1418.60 real         5.74 user       181.64 sys
      508.67 real         2.67 user       119.66 sys

     1361.19 real         5.97 user       176.94 sys
     1327.63 real         5.72 user       179.60 sys
     1376.16 real         6.33 user       179.72 sys
     1356.47 real         6.07 user       180.24 sys
      462.67 real         2.30 user       119.18 sys

I'm using the newfs defaults, but dirsize is recalculated as the
filesystem fills if the appropriate value is larger than what is
calculated from the parameters set by newfs.

I suspect the problem is the large bimodal distribution in file size in
my benchmark, with zillions of little files, but also a number of large
package files and source distfiles.  The large files muck up the dirsize
calculation because they are actually distributed across multiple
cylinder groups and only the first maxbpg blocks are allocated in the
original cylinder group.  This would be easy to account for in the
avgfilesize * avgfpdir formula, but I don't know how to handle this in
the curdirsize formula (other than the degenerate case where most files
are larger than maxbpg).

Since I probably won't have time to get anything different tested before
the -CURRENT code freeze, do you have any objections if I just MFC the
code that I previously committed to -CURRENT?  It certainly seems to
perform better than the original code which is still in 4-STABLE.