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memory_trolling(5)
NAME
memory_trolling - Proactively locates and scrubs correctable memory errors
SYNOPSIS
/etc/sysconfigtab
vm:
vm_troll_percent = percent_rate
DESCRIPTION
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
system, 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
variable 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
percentage 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
trolling. 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:
vm:
vm_troll_percent=percent_rate
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
...cpu...
...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
command:
# /sbin/sysconfig -r vm vm_troll_percent=percent_rate
Repeat step 3 and note any change in the value of sy under the cpu
heading.
A system time (sy) increase of one or less represents negligible
performance 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.
MESSAGES
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
recommended.
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
message 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
indicate 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
terminal, 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
continues 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.
EXAMPLES
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/sh
/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
FILES
/etc/sysconfigtab
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.
/sbin/sysconfig
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.
SEE ALSO
Commands: sys_attrs_vm(5), sysconfig(8), sysconfigdb(8), and vmstat(1)
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