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Who Me Too'd this topic
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06-21-2012 07:48 PM
06-21-2012 07:48 PM
Hyperthreading and VMS
Hi,
When the first hyperthread-capable Itanium servers came out with VMS support, I remember a comment from a VMS engineering presentation that said each CPU thread would show up as a separate CPU on VMS, and need a licence accordingly. This would have been with VMS 8.3 I believe.
A BL860c i2 server blade here has a single, quad-core Tukwila processor with hyperthreading turned on. VMS shows 8 CPUs (as expected):
System: HOCD04, HP Integrity BL860c i2 (1.60GHz/5.0MB)
CPU ownership sets:
Active 0-7
Configure 0-7
CPU state sets:
Potential 0-7
Autostart 0-7
Powered Down None
Not Present None
Hard Excluded None
Failover None
The system has a 4-unit HAOE licence:
------- Product ID -------- ---- Rating ----- -- Version --
Product Producer Units PCL Activ Version Release Termination
OPENVMS-I64-HAOE HP 4 1 0 0.0 (none) (none)
so I expected that only half of the available 8 CPUs would be used.
As a test, set off 8 processes which run in a tight CPU loop at priority 0. Somewhat to my surprise, MONITOR SYSTEM shows 8 processes in CUR state and 800% CPU busy. Which implies that the 4-unit licence above is allowing all 8 CPUs to operate.
Rather more curiously MONITOR PROCESS/TOPCPU shows 8 processes each consuming 50% of CPU:
OpenVMS Monitor Utility
TOP CPU TIME PROCESSES
on node HOCD04
22-JUN-2012 12:21:52.93
0 25 50 75 100
+ - - - - + - - - - + - - - - + - - - - +
2200029B Jeremy6 50 xxxxxxxxxxxxxxxxxxx
2200029D Jeremy8 50 xxxxxxxxxxxxxxxxxxx
2200029C Jeremy7 50 xxxxxxxxxxxxxxxxxxx
22000299 Jeremy4 50 xxxxxxxxxxxxxxxxxxx
22000298 Jeremy3 50 xxxxxxxxxxxxxxxxxxx
22000297 Jeremy2 50 xxxxxxxxxxxxxxxxxxx
22000296 Jeremy1 50 xxxxxxxxxxxxxxxxxxx
2200029A Jeremy5 50 xxxxxxxxxxxxxxxxxxx
which would imply there's some sort of recognition that a hyperthread CPU is only half a core!
Can some explain how VMS treats hyperthreaded CPUs, both from a scheduling perspective and a licencing perspective? (I know there's some doubt about how helpful hyperthreading might be for performance, and on other systems I've been turning it off. My findings today came about by accident.)
Thanks,
Jeremy Begg
- Tags:
- License