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memory_trolling(5)					   memory_trolling(5)


  Memory Trolling - Proactively	locates	and scrubs correctable memory errors



       vm_troll_percent	=  percent_rate


  The operating	system handles memory errors with a just-in-time scrubbing
  model, where correctable errors are scrubbed when encountered	by the
  operating system or an application.  To enhance this capability, a trigger
  mechanism, called the	memory troller proactively locates and scrubs
  correctable memory errors.  The memory troller systematically	reads each
  memory location. If it discovers a correctable memory	error, it triggers
  the just-in-time scrubbing mechanism.

  Since	the memory troller reads all memory available to the operating sys-
  tem, it might	also discover uncorrectable memory errors, which would lead
  to a unrecoverable machine check. To avoid this, the operating system
  recognizes that the machine check resulted from memory trolling, dismisses
  the error, and continues normal operation. The memory	troller	then causes
  the memory page containing the uncorrectable error to	be marked as a bad
  page.	If the bad page	is free	(or when it becomes free) it is	then mapped
  out so it will not be	reused.

  Enabling, Disabling, and Tuning Memory Trolling

  For systems supported	by the memory troller, use the vm_troll_percent	vari-
  able to enable, disable, and tune the	trolling rate. This parameter is part
  of the kernel's vm subsystem.	The trolling rate is expressed as a percen-
  tage of the system's total memory trolled per	hour and can be	changed	at
  any time. Valid troll	rate settings are as follows:

  Default value: 4 percent per hour
      This value is used by default if you do not specify any value for
      vm_troll_percent.	At this	default	rate, each 8 kilobyte memory page is
      trolled once every 24 hours.

  Disable value: 0 (zero)
      A	value of zero disables the memory troller.

  Range: 1 - 100 percent
      The troll	rate is	set to the specified percentage	of   memory to troll
      per hour.	For example, a 50 percent troll	rate reads half	 the total
      memory in	one hour. After	all memory is read, the	troller	starts a new
      pass at the beginning of memory.

  Accelerated trolling:	101 percent
      Any value	greater	than 100 percent invokes one-pass accelerated trol-
      ling. All	memory is trolled at a rate of approximately 6000 8 kilobyte
      pages per	second,	then trolling is disabled. This	mode is	intended for
      trolling all memory quickly during off   peak hours. For example,	on a
      GS320 system with	32 processors and 128 gigabytes	of memory, one-pass
      accelerated trolling takes approximately five minutes.

  Use the following command to display the current value of vm_troll_percent
  (the troll rate):

       #  /sbin/sysconfig -q vm	vm_troll_percent

  You can override the default troll rate by adding the	following lines	to
  the /etc/sysconfigtab	file:


  The percent_rate variable is the troll rate as described previously.	Use
  the sysconfigdb command to add entries to the	/etc/sysconfigtab file,	as
  described in the sysconfigdb(8) reference page. The new rate takes effect
  on the next system boot.

  You can enable, disable, or change the troll rate at any time	using the
  following command:

       # /sbin/sysconfig -r vm vm_troll_percent=percent_rate

  The variable is the troll rate as described previously. Only the superuser
  (root) or a user authorized by division of privileges	(dop) can use this
  command.  (Refer to the dop(8) reference page	for information	on sharing
  superuser privileges.)

  See MESSAGES for information on configuration	messages

  Controlling the Use of System	Resources

  Low trolling rates, such as the 4 percent default, have negligible impact
  on system performance. Processor usage for memory trolling increases as the
  troll	rate is	increased. To approximate the performance overhead, use	the
  following procedure:

   1.  Log in as root or become	superuser.

   2.  Choose a	time when the system is	idle and disable the memory troller
       using the following command:

	    #  /sbin/sysconfig -r vm vm_troll_percent=0

   3.  To establish a performance baseline, run	the following command with
       the memory troller disabled:

	    #  vmstat 1
	    ...us  sy  id...
	    ...	1   1  98...

   4.  In the command output, note the system time, labeled sy under the cpu
       heading.	Adjust the value of vm_troll_percent using the following com-

	    #  /sbin/sysconfig -r vm vm_troll_percent=percent_rate

       Repeat step 3 and note any change in the	value of sy under the cpu

  A system time	(sy) increase of one or	less represents	negligible perfor-
  mance	cost.  Repeat the procedure, adjusting the percent value of
  vm_troll_percent until the performance cost is acceptable.

  For example, a GS320 system with 32 processors and 128 GB of memory will
  show approximately 25	percent	of system time during one-pass accelerated
  trolling. The	same system at the 4 percent default troll rate	will show one
  percent or less system time.


  Configuration	Messages

  If  the memory troller does not support your system, the following error is
  displayed on your terminal when you attempt to configure the memory troller
  using	/sbin/sysconfig:

       vm_configure: Memory Trolling not supported on this system.

  You can disable trolling using the following command:

       #  /sbin/sysconfig -r vm	vm_troll_percent=0

  The following	warning	message	is displayed on	your terminal when the
  preceding command is executed:

       vm_configure: shutting down memory troller.
       [WARNING: disabling the memory troller is not recommended on
       this system.]

  This message notifies	you that permanently disabling memory trolling is not

  Informational	Messages

  The following	messages provide information about events associated with
  memory troller operation. These messages do not indicate a failure in	the
  memory troller:

    +  If a memory page	containing a uncorrectable error was located by	the
       memory troller and the bad page will be mapped out, the following mes-
       sage is displayed:

	    Memory Troller: bad	page found (address = 0x################)

    +  In addition to the bad page found... message, machine check messages
       similar to the following	are displayed on the system's console when
       the memory troller encounters a bad page:

	    25-Mar-2000	17:24:25 [700] CPU machine check/exception - CPU 0
	    25-Mar-2000	17:24:25 [700] CPU machine check/exception - CPU 18

       These messages come from	the event notification subsystem. They indi-
       cate that the machine checks resulting from the memory troller reading
       the bad page have been entered into the binary error log.

  Error	Messages

  If any of the	following error	messages are displayed on the console termi-
  nal, a malfunction has occurred in the memory	troller	and you	must contact
  your technical support organization.

    +  VM_CONFIGURE: Memory Trolling is	currently disabled on this system

       The memory troller has been disabled due	to a fatal error.

    +  adjust_troll_quantity: null MAD pointer,	disabling troller

       A fatal internal	error has occurred, the	troller	is disabled.

    +  adjust_troll_quantity: invalid troll_percent 0 defaulting to 4 percent

       The troller is active, but the troll rate is zero. The troller contin-
       ues operating, but at the default troll rate. This is a serious error.

    +  vm_memory_troller: CPU #	vmmt_get_mad() failed, disabling troller

       A fatal internal	error has occurred, the	troller	is disabled.

    +  vm_memory_troller: MAD #	invalid	state [#], shutting down

       A fatal internal	error has occurred, the	troller	is disabled.


  The following	examples demonstrate typical command use and settings for the
  memory troller:

   1.  To schedule one-pass accelerated	trolling at off	peak hours, use	the
       following procedure:

	a.  Create a shell script named	/usr/local/fast_troll.sh containing
	    the	following lines:


		 /sbin/sysconfig -r vm vm_troll_percent=101

	b.  Using the following	commands, set the file owner and permissions
	    of /usr/local/fast_troll.sh:

		 #  chown root /usr/local/fast_troll.sh
		 #  chmod 744 /usr/local/fast_troll.sh

	c.  Use	the cron facility to schedule execution	of the shell script
	    as root user at the	desired	time. (Refer to	the cron(8) reference
	    page for more information.)

   2.  The following command demonstrates how you can set trolling at a	more
       aggressive rate of 50 percent per hour:

	    #  /sbin/sysconfig -r vm vm_troll_percent=50

       As such dynamic changes are not recorded	in the /etc/sysconfigtab
       file, this setting will not persist across a reboot.

   3.  The following method describes how you use a stanza file	to change the
       value of	vm_troll_percent to 10 so that the change is updated in	the
       kernel immediately and also persists across a reboot:

	a.  Create a stanza file containing the	following lines:

		 vm:	 vm_troll_percent=10

	    Save this file as /tmp/vm_troller.stanza.

	b.  Use	the following command to merge the stanza in the
	    /etc/sysconfigtab file:

		 #/sbin/sysconfigdb -a -f /tmp/vm_troller.stanza vm


      The configuration	database file in which you specify the value of
      vm_troll_percent under the vm attributes.	See the	sysconfigtab(4)
      reference	page for more information.

      The command that you use to dynamically set the value of
      vm_troll_percent under the vm attributes in the /etc/sysconfigtab	file.
      See the sysconfig(8) reference page for more information.


  Commands: sys_attrs_vm(5), sysconfig(8), sysconfigdb(8), and vmstat(1)