LXR linux/Documentation/powerpc/ppc

   1                 Information about /proc/ppc_htab
   2=====================================================================
   3
   4This document and the related code was written by me (Cort Dougan), please
   5email me (cort@fsmlabs.com) if you have questions, comments or corrections.
   6
   7Last Change: 2.16.98
   8
   9This entry in the proc directory is readable by all users but only
  10writable by root.
  11
  12The ppc_htab interface is a user level way of accessing the
  13performance monitoring registers as well as providing information
  14about the PTE hash table.
  15
  161. Reading
  17
  18  Reading this file will give you information about the memory management
  19  hash table that serves as an extended tlb for page translation on the
  20  powerpc.  It will also give you information about performance measurement
  21  specific to the cpu that you are using.
  22
  23  Explanation of the 604 Performance Monitoring Fields:
  24    MMCR0 - the current value of the MMCR0 register
  25    PMC1
  26    PMC2 - the value of the performance counters and a
  27           description of what events they are counting
  28           which are based on MMCR0 bit settings.
  29  Explanation of the PTE Hash Table fields:
  30
  31    Size - hash table size in Kb.
  32    Buckets -  number of buckets in the table.
  33    Address - the virtual kernel address of the hash table base.
  34    Entries - the number of ptes that can be stored in the hash table.
  35    User/Kernel - how many pte's are in use by the kernel or user at that time.
  36    Overflows - How many of the entries are in their secondary hash location.
  37    Percent full - ratio of free pte entries to in use entries.
  38    Reloads - Count of how many hash table misses have occurred
  39              that were fixed with a reload from the linux tables.
  40              Should always be 0 on 603 based machines.
  41    Non-error Misses - Count of how many hash table misses have occurred
  42              that were completed with the creation of a pte in the linux
  43              tables with a call to do_page_fault().
  44    Error Misses - Number of misses due to errors such as bad address
  45              and permission violations.  This includes kernel access of
  46              bad user addresses that are fixed up by the trap handler.
  47
  48  Note that calculation of the data displayed from /proc/ppc_htab takes
  49  a long time and spends a great deal of time in the kernel.  It would
  50  be quite hard on performance to read this file constantly.  In time
  51  there may be a counter in the kernel that allows successive reads from
  52  this file only after a given amount of time has passed to reduce the
  53  possibility of a user slowing the system by reading this file.
  54
  552. Writing
  56
  57  Writing to the ppc_htab allows you to change the characteristics of
  58  the powerpc PTE hash table and setup performance monitoring.
  59
  60  Resizing the PTE hash table is not enabled right now due to many
  61  complications with moving the hash table, rehashing the entries
  62  and many many SMP issues that would have to be dealt with.
  63
  64  Write options to ppc_htab:
  65  
  66   - To set the size of the hash table to 64Kb:
  67
  68      echo 'size 64' > /proc/ppc_htab
  69
  70     The size must be a multiple of 64 and must be greater than or equal to
  71     64.
  72
  73   - To turn off performance monitoring:
  74
  75      echo 'off' > /proc/ppc_htab
  76
  77   - To reset the counters without changing what they're counting:
  78
  79      echo 'reset' > /proc/ppc_htab
  80
  81     Note that counting will continue after the reset if it is enabled.
  82
  83   - To count only events in user mode or only in kernel mode:
  84
  85      echo 'user' > /proc/ppc_htab
  86       ...or...
  87      echo 'kernel' > /proc/ppc_htab
  88
  89     Note that these two options are exclusive of one another and the
  90     lack of either of these options counts user and kernel.
  91     Using 'reset' and 'off' reset these flags.
  92
  93   - The 604 has 2 performance counters which can each count events from
  94     a specific set of events.  These sets are disjoint so it is not
  95     possible to count _any_ combination of 2 events.  One event can
  96     be counted by PMC1 and one by PMC2.
  97
  98     To start counting a particular event use:
  99
 100      echo 'event' > /proc/ppc_htab
 101
 102     and choose from these events:
 103
 104     PMC1
 105     ----
 106      'ic miss' - instruction cache misses
 107      'dtlb' - data tlb misses (not hash table misses)
 108
 109     PMC2
 110     ----
 111      'dc miss' - data cache misses
 112      'itlb' - instruction tlb misses (not hash table misses)
 113      'load miss time' - cycles to complete a load miss
 114
 1153. Bugs
 116
 117  The PMC1 and PMC2 counters can overflow and give no indication of that
 118  in /proc/ppc_htab.
 119