patch-2.1.67 linux/Documentation/powerpc/ppc_htab.txt

• Lines: 124
• Date: Sat Nov 29 10:33:18 1997
• Orig file: v2.1.66/linux/Documentation/powerpc/ppc_htab.txt
• Orig date: Thu Sep 4 17:07:29 1997

diff -u --recursive --new-file v2.1.66/linux/Documentation/powerpc/ppc_htab.txt linux/Documentation/powerpc/ppc_htab.txt
@@ -1,25 +1,47 @@
                  Information about /proc/ppc_htab
 =====================================================================
 
+This document and the related code was written by me (Cort Dougan), please
+email me (cort@cs.nmt.edu) if you have questions, comments or corrections.
+
 This entry in the proc directory is readable by all users but only
 writable by root.
 
+The ppc_htab interface is a user level way of accessing the
+performance monitoring registers as well as providing information
+about the PTE hash table.
 
 1. Reading
 
   Reading this file will give you information about the memory management
   hash table that serves as an extended tlb for page translation on the
-  powerpc.
+  powerpc.  It will also give you information about performance measurement
+  specific to the cpu that you are using.
 
-  Explaination of the fields:
+  Explanation of the 604 Performance Monitoring Fields:
+    MMCR0 - the current value of the MMCR0 register
+    PMC1
+    PMC2 - the value of the performance counters and a
+           description of what events they are counting
+           which are based on MMCR0 bit settings.
+  Explanation of the PTE Hash Table fields:
 
     Size - hash table size in Kb.
     Buckets -  number of buckets in the table.
-    Addess - the virtual kernel address of the hash table base.
+    Address - the virtual kernel address of the hash table base.
     Entries - the number of ptes that can be stored in the hash table.
     User/Kernel - how many pte's are in use by the kernel or user at that time.
     Overflows - How many of the entries are in their secondary hash location.
     Percent full - ratio of free pte entries to in use entries.
+    Reloads - Count of how many hash table misses have occurred
+              that were fixed with a reload from the linux tables.
+              Should always be 0 on 603 based machines.
+    Non-error Misses - Count of how many hash table misses have occurred
+              that were completed with the creation of a pte in the linux
+              tables with a call to do_page_fault().
+    Error Misses - Number of misses due to errors such as bad address
+              and permission violations.  This includes kernel access of
+              bad user addresses that are fixed up by the trap handler.
 
   Note that calculation of the data displayed from /proc/ppc_htab takes
   a long time and spends a great deal of time in the kernel.  It would
@@ -30,21 +52,70 @@
 
 2. Writing
 
-  Writing to ppc_htab is not yet allowed.
+  Writing to the ppc_htab allows you to change the characteristics of
+  the powerpc PTE hash table and setup performance monitoring.
+
+  Resizing the PTE hash table is not enabled right now due to many
+  complications with moving the hash table, rehashing the entries
+  and many many SMP issues that would have to be dealt with.
 
   Write options to ppc_htab:
   
    - To set the size of the hash table to 64Kb:
 
-      echo 'size 64' > /dev/ppc_htab
+      echo 'size 64' > /proc/ppc_htab
 
      The size must be a multiple of 64 and must be greater than or equal to
      64.
 
+   - To turn off performance monitoring:
+
+      echo 'off' > /proc/ppc_htab
+
+   - To reset the counters without changing what they're counting:
+
+      echo 'reset' > /proc/ppc_htab
+
+     Note that counting will continue after the reset if it is enabled.
+
+   - To count only events in user mode or only in kernel mode:
+
+      echo 'user' > /proc/ppc_htab
+       ...or...
+      echo 'kernel' > /proc/ppc_htab
+
+     Note that these two options are exclusive of one another and the
+     lack of either of these options counts user and kernel.
+     Using 'reset' and 'off' reset these flags.
+
+   - The 604 has 2 performance counters which can each count events from
+     a specific set of events.  These sets are disjoint so it is not
+     possible to count _any_ combination of 2 events.  One event can
+     be counted by PMC1 and one by PMC2.
+
+     To start counting a particular event use:
+
+      echo 'event' > /proc/ppc_htab
 
+     and choose from these events:
 
+     PMC1
+     ----
+      'ic miss' - instruction cache misses
+      'dtlb' - data tlb misses (not hash table misses)
 
+     PMC2
+     ----
+      'dc miss' - data cache misses
+      'itlb' - instruction tlb misses (not hash table misses)
+      'load miss time' - cycles to complete a load miss
 
+3. Bugs
 
+  Doing a 'less' or 'more' on ppc_htab results in a segmentation violation.
+  I'm not sure of the cause but in the mean time 'cat' works adequately for
+  reading the file.
 
+  The PMC1 and PMC2 counters can overflow and give no indication of that
+  in /proc/ppc_htab.