A fix is available
APAR status
Closed as program error.
Error description
When a guest is performing heavy I/O it may not be limited to the guest's ABSOLUTE LIMITHARD SETTING. Not limiting a guest accurately can negatively impact other guest's consumption capability, particularly in a CPUPOOL.
Local fix
local patch is available upon request
Problem summary
**************************************************************** * USERS AFFECTED: All users of SET SHARE LIMITHARD or CPU POOL * * limiting of guest CPU utilization * **************************************************************** * PROBLEM DESCRIPTION: * **************************************************************** * RECOMMENDATION: APPLY PTF * **************************************************************** SHARE with the LIMITHARD option and RESPOOL (CPUPOOL) limiting may not work correctly for all work loads. A guest with a hard limit (SET SHARE ... LIMITHARD) or that is a member of a CPU pool with a CPU limit (LIMITHARD or CAPACITY) might not be limited appropriately. While this could occur for several reasons, the most likely is that the guest has a high degree of I/O activity. While I/O is in progress, a guest holds a share of a lock, which prevents it from being added to the Limit List. As long as at least one guest I/O operation is in flight this condition will prevail. Eventually, when there is no active I/O, the guest will be limited, potentially for a long period of time, during which it will be completely unresponsive. In scenarios, such as the one described above, CP work dispatched on a guest VMDBK requires a critical lock, or more than one critical lock. Each time one of the locks is obtained, a count of critical processes (VMDCTCRT) is incremented. It is decremented when the lock is released. The VMDBK is not limited when that critical count is non-zero so that release of the lock isn't delayed for a long time while the guest is limited. When a guest is doing things that require much CP involvement and that CP work frequently requires critical locks, the critical count can stay non-zero for an extended amount of time. If that happens and the guest is limited by the SHARE LIMITHARD option or by RESPOOL limiting, the VMDBK may be withheld from the limit list even though it is due to be limited. Eventually, when the count goes to zero, the VMDBK will be limited for an extended amount of time to make up for the excess CPU it used when it should have been limited. This can cause erratic response times from the guest and can lead to guest time-outs.
Problem conclusion
The dispatcher and scheduler are changed to dispatch CP work on behalf of a guest VMDBK that is being otherwise limited. The CP work will be allowed to execute but the VMDBK will not be run under SIE. To accomplish this the actual addition of a VMDBK to the limit list is moved from the scheduler to the dispatcher. The dispatcher will only add the VMDBK to the limit list when no more CP work is stacked on it. Also, the VMDBK is temporarily taken off the limit list if it is being made ready because CP work has been stacked on it.
Temporary fix
FOR RELEASE VM/ESA CP/ESA R640 : PREREQ: VM65615 CO-REQ: NONE IF-REQ: NONE FOR RELEASE VM/ESA CP/ESA R710 : PREREQ: VM65715 CO-REQ: NONE IF-REQ: NONE
Comments
×**** PE20/06/10 FIX IN ERROR. SEE APAR VM66400 FOR DESCRIPTION
APAR Information
APAR number
VM66154
Reported component name
VM CP
Reported component ID
568411202
Reported release
630
Status
CLOSED PER
PE
NoPE
HIPER
NoHIPER
Special Attention
NoSpecatt / Xsystem
Submitted date
2018-04-25
Closed date
2019-05-02
Last modified date
2020-12-16
APAR is sysrouted FROM one or more of the following:
APAR is sysrouted TO one or more of the following:
UM35444 UM35445
Modules/Macros
HCPCPA HCPCPU HCPDSP HCPDSW HCPLMC HCPSCH HCPSCI HCPSCK HCPSRN HCPSTK HCPSTL HCPSTM HCPVMDBK
Fix information
Fixed component name
VM CP
Fixed component ID
568411202
Applicable component levels
Fix is available
Select the PTF appropriate for your component level. You will be required to sign in. Distribution on physical media is not available in all countries.
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Document Information
Modified date:
12 January 2021