NFS Tweaks for FreeBSD 4.6 (or Later)

• A description of the tweaks:

  Daniel Ellard, Margo Seltzer. NFS Tricks and Benchmarking Traps (PDF, HTML)
  Published in the proceedings of the FREENIX Technical Conference, San Antonio, Texas. June 2003.

• A context diff for the change to improve the nfsheur hash table, relative to the 4.8-RELEASE.

  The two changes are to increase NUM_HEURISTIC, and increase the number of probes attempted before a search of the table is abandoned. You could make the max probes as high as NUM_HEURISTIC, if you want to trade time for space. My choice of constants is arbitrary; maybe they should be config options so people with really big servers can set it to whatever they like when they build their kernels.

  Along with this is a change in the hash function that's needed if NUM_HEURISTIC isn't a power of two (the current code uses a bitmask to compute mod NUM_HEURISTIC).

• The modified source code for full set of NFS tweaks described in the paper:
  • nfs/nfs_serv.c

    The changes are easier to see as a context diff from the original 4.6.2p4 source.

  • nfs/nfs_tweak.c

  • nfs/nfs_tweak.h

• Source code for the N-CSCAN scheduler:
  • ufs/ufs/ufs_disksubr.c

    The changes are easier to see as a context diff from the original 4.6.2p4 source.

Notes

• None of these changes will have any effect unless the kernel is compiled with -DNFS_TWEAK.

• The nfs_tweak.c code refers to variable nfst_CscanMode, which controls whether CSCAN or N-CSCAN disk scheduling is used. Therefore, if wish to use only the NFS tweaks but not the N-CSCAN code, you will need to move the definition of nfst_CscanMode from ufs/ufs/ufs_disksubr.c to nfs/nfs_tweak.c.

• Use sysctl to set the values for the variables that control the NFS tweak parameters (or to disable all of the tweaks). The relevant sysctl variables are defined and in nfs_tweak.c and ufs_disksubr.c. All of their names begin with vfs.nfs.nfst via the sysctl interface (or nfst_ in the source code).