Hello everyone,
I want to implement the cpu_relax() function in order to use it inside loops. However before, I would like to know if the fpc compiler is doing an optimization of this kind.
Kinds regards, Matias.
Mailing lists / fpc-pascal / 2017-10
cpu_relax() in freepascal
5 messages · 3 people · –
- Matias Vara 3
- Adriaan van Os 1
- Karoly Balogh (Charlie/SGR 1
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Hello,
I think it would be better if I put my question in context. In order to tell the processor that we are in a loop, the pause instruction could be used (also I think the "rep nop" produces the same opcode). This tells the processor that we are in a loop so it improves the access to the cache and the branching mechanism. My question is if the compiler is doing any improvement of this kind in a loop or I should write my own "pause" procedure/macro. For more info
https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops
Cheers!
2017-10-29 16:59 GMT+01:00 Matias Vara :
Show quoted text 8 lines
Hello everyone,
I want to implement the cpu_relax() function in order to use it inside
loops. However before, I would like to know if the fpc compiler is doing an
optimization of this kind.
Kinds regards, Matias.
I want to implement the cpu_relax() function in order to use it inside loops. However before, I would like to know if the fpc compiler is doing an optimization of this kind.
I don't think one would you use that "inside a loop" or "to improve branching" but rather inside a wait-loop where constant polling, without regular pausing, keeps the processor unnecessarily busy. The operating system has calls for that purpose (e.g. usleep, see <https://linux.die.net/man/3/usleep>). That is not up to the compiler.
Regards,
Adriaan van Os
I think it would be better if I put my question in context. In order to tell the…
Hi,
Matias Vara wrote 9 lines
Hello, I think it would be better if I put my question in context. In
order to tell the processor that we are in a loop, the pause instruction
could be used (also I think the "rep nop" produces the same opcode).
This tells the processor that we are in a loop so it improves the access
to the cache and the branching mechanism. My question is if the compiler
is doing any improvement of this kind in a loop or I should write my own
"pause" procedure/macro. For more info
https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops
This feature seems to be a compiler intrinsic at least in the Intel C compiler. We don't have anything similar, so you have to fall back either to an asm procedure or a line of inline assembler. The compiler is also not doing an optimization similar to this. Free Pascal's inline assembler supports the PAUSE instruction though.
But unless you plan to use this in a multithreaded spinloop because you're loading multiple cores of the CPU, it's better to let the operating system handle the wait. This instruction is certainly not the way to fix a "my app uses 100% CPU" scenario, just saying.
Charlie
Hi and thanks for the answers,
This feature seems to be a compiler intrinsic at least in the Intel C
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compiler. We don't have anything similar, so you have to fall back either
to an asm procedure or a line of inline assembler. The compiler is also
not doing an optimization similar to this. Free Pascal's inline assembler
supports the PAUSE instruction though.
good to know that.
Show quoted text 6 lines
But unless you plan to use this in a multithreaded spinloop because you're
loading multiple cores of the CPU, it's better to let the operating system
handle the wait. This instruction is certainly not the way to fix a "my
app uses 100% CPU" scenario, just saying.
Charlie
I am currently investigating the use of this instruction in my kernel. I have wait-loops when accessing shared resources in a multicore system so that could be a place to use it.
Also, I have situations in which the core just spends time in a loop so this instruction may help to reduce energy consumption. All this work is however very experimental.
Matias