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Information about /proc/ppc_htab
This document and the related code was written by me (Cort Dougan), please
email me ( 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
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
- 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
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:
'ic miss' - instruction cache misses
'dtlb' - data tlb misses (not hash table misses)
'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.