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Why UOPS_RETIRED.ALL greater than UOPS_RETIRED.RETIRE_SLOTS

Hello 

Could you explain me a difference between those two events UOPS_RETIRED.ALL_PS and UOPS_RETIRED.RETIRE_SLOTS_PS on Sandy Bridge?

would expect that those events should give approximately the same numbers, since number of used slots should agree we with number of retired uops during period of time. Data below shows that number of used retirement slots is lesser by ~20%  than number ups retired.

Is it possible that uops retired w/o using slot? 

UOPS_RETIRED.ALL_PS - This event counts the number of micro-ops retired.

UOPS_RETIRED.RETIRE_SLOTS_PS - This event counts the number of retirement slots used each cycle.  There are potentially 4 slots that can be used each cycle - meaning, 4 micro-ops or 4 instructions could retire each cycle.  This event is used in determining the 'Retiring' category of the Top-Down pipeline slots characterization.

Hardware Events

Hardware Event Type Hardware Event Count    Hardware Event Sample Count Events Per Sample

CPU_CLK_UNHALTED.THREAD_P   49,458,074,187  8,243   2000003
INST_RETIRED.ANY_P  25,656,038,484  4,276   2000003
UOPS_RETIRED.ALL_PS 35,772,053,658  5,962   2000003
UOPS_RETIRED.RETIRE_SLOTS_PS    29,574,044,361  4,929   2000003

 

Thanks,

Alexander

 

 

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