On Tue, 12 Mar 2019 09:32:18 -0700, Charles Mills wrote:
>Yes, STCK guarantees a unique value. If the clock has not ticked since the
>last STCK, the CPU has no choice but to spin until it does. STCKE has smaller
>"ticks" and so has less of (or no) need for a spin. STCKF is just like STCK
>except that it does not guarantee a unique value, and so there is never a need
>to spin, and so it is a "fast" instruction. If all you need is "the time" and
>not a unique timestamp, always use STCKF. It is a one-character change to your
>program and may speed it up considerably.
Long ago, I understood that each CPU had its own TOD clock, for fault
tolerance. fact, I studied
the elaborate code in VM CP to line up all the clocks at the starting line
until waving the green flag.
However, recently I read:
>IBM ® z/Architecture Principles of Operation SA22-7832-10
>Chapter 4. Control
> Timing
...
In a multiprocessing configuration, a single TOD clock is shared by all
CPUs.
There's a lot of text and a figure with revision bars that could summarize as:
"Don't use STCK and STCKE in the same program or interacting programs."
-- gil
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