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1275 |
phoenix |
Information about /proc/ppc_htab
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=====================================================================
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This document and the related code was written by me (Cort Dougan), please
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email me (cort@fsmlabs.com) if you have questions, comments or corrections.
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Last Change: 2.16.98
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This entry in the proc directory is readable by all users but only
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writable by root.
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The ppc_htab interface is a user level way of accessing the
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performance monitoring registers as well as providing information
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about the PTE hash table.
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1. Reading
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Reading this file will give you information about the memory management
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hash table that serves as an extended tlb for page translation on the
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powerpc. It will also give you information about performance measurement
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specific to the cpu that you are using.
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Explanation of the 604 Performance Monitoring Fields:
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MMCR0 - the current value of the MMCR0 register
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PMC1
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PMC2 - the value of the performance counters and a
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description of what events they are counting
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which are based on MMCR0 bit settings.
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Explanation of the PTE Hash Table fields:
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Size - hash table size in Kb.
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Buckets - number of buckets in the table.
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Address - the virtual kernel address of the hash table base.
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Entries - the number of ptes that can be stored in the hash table.
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User/Kernel - how many pte's are in use by the kernel or user at that time.
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Overflows - How many of the entries are in their secondary hash location.
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Percent full - ratio of free pte entries to in use entries.
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Reloads - Count of how many hash table misses have occurred
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that were fixed with a reload from the linux tables.
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Should always be 0 on 603 based machines.
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Non-error Misses - Count of how many hash table misses have occurred
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that were completed with the creation of a pte in the linux
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tables with a call to do_page_fault().
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Error Misses - Number of misses due to errors such as bad address
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and permission violations. This includes kernel access of
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bad user addresses that are fixed up by the trap handler.
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Note that calculation of the data displayed from /proc/ppc_htab takes
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a long time and spends a great deal of time in the kernel. It would
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be quite hard on performance to read this file constantly. In time
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there may be a counter in the kernel that allows successive reads from
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this file only after a given amount of time has passed to reduce the
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possibility of a user slowing the system by reading this file.
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2. Writing
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Writing to the ppc_htab allows you to change the characteristics of
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the powerpc PTE hash table and setup performance monitoring.
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Resizing the PTE hash table is not enabled right now due to many
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complications with moving the hash table, rehashing the entries
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and many many SMP issues that would have to be dealt with.
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Write options to ppc_htab:
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- To set the size of the hash table to 64Kb:
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echo 'size 64' > /proc/ppc_htab
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The size must be a multiple of 64 and must be greater than or equal to
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64.
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- To turn off performance monitoring:
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echo 'off' > /proc/ppc_htab
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- To reset the counters without changing what they're counting:
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echo 'reset' > /proc/ppc_htab
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Note that counting will continue after the reset if it is enabled.
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- To count only events in user mode or only in kernel mode:
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echo 'user' > /proc/ppc_htab
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...or...
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echo 'kernel' > /proc/ppc_htab
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Note that these two options are exclusive of one another and the
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lack of either of these options counts user and kernel.
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Using 'reset' and 'off' reset these flags.
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- The 604 has 2 performance counters which can each count events from
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a specific set of events. These sets are disjoint so it is not
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possible to count _any_ combination of 2 events. One event can
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be counted by PMC1 and one by PMC2.
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To start counting a particular event use:
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echo 'event' > /proc/ppc_htab
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and choose from these events:
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PMC1
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----
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'ic miss' - instruction cache misses
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'dtlb' - data tlb misses (not hash table misses)
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PMC2
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----
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'dc miss' - data cache misses
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'itlb' - instruction tlb misses (not hash table misses)
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'load miss time' - cycles to complete a load miss
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3. Bugs
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The PMC1 and PMC2 counters can overflow and give no indication of that
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in /proc/ppc_htab.
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