• Mike Hibler's avatar
    The latest installment of Mike's Fine Mountd Hackary (MFMH). · 89fe803f
    Mike Hibler authored
    Previously on MFMH:
    
     * Our hero added incremental kernel export updates to mountd so
       that a single update would finish within his lifetime,
    
     * Hero subsequently discovers that an incremental update still
       takes 8 seconds on the Emulab Mothership,
    
     * Hero discovers that remaining time is spent NOT in kernel operations
       or even complex calculations of orbital mechanics, but rather in simple
       parsing of text from a 1300 line file.
    
     * Hero gives up in despair.
    
    This week on MFMH:
    
     * Our revitalized hero applies gprof-fu to the problem and
       discovers that 7.9 seconds of the 8 second /etc/exports parse time
       is spent stat()ing /etc/nsswitch.conf 2,392,057 times.
    
     * "WtF!", exclaims our hero, "that is 27.3 stats for every text
       character in the file!"
    
     * Problem is traced to the fact that we do "-maproot=root" for each
       of the 1344 exports and every parse of that option looks up root
       in the password file and determines which of the 1784 groups in
       /etc/group root belongs to. Each of those 1344 * 1784 "get group
       entry" calls, stats /etc/nsswitch.conf to see if the file has
       changed.
    
     * "Bite me!", sez Mike, and he adds a CACHE_ROOT_CREDS option to
       mountd so that the lookup is only done on the very first instance
       of -maproot=root.
    
    A single incremental update of exports now takes 0.75 seconds.
    89fe803f