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+ Information about /proc/ppc_htab
+This document and the related code was written by me (Cort Dougan), please
+email me (cort@fsmlabs.com) if you have questions, comments or corrections.
+Last Change: 2.16.98
+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. It will also give you information about performance measurement
+ specific to the cpu that you are using.
+ 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.
+ 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
+ be quite hard on performance to read this file constantly. In time
+ there may be a counter in the kernel that allows successive reads from
+ this file only after a given amount of time has passed to reduce the
+ possibility of a user slowing the system by reading this file.
+2. Writing
+ 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' > /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
+ The PMC1 and PMC2 counters can overflow and give no indication of that
+ in /proc/ppc_htab.