[gem5-users] Doubling system clock freq doesn't halve simulated time?

[Heng Yu]

Hi,

I am playing in the SE mode, with script 'GEM5_PATH/configs/learning_gem5/part2/simple_memobj.py', where I altered frequencies between:

# Set the clock fequency of the system (and all of its children)
system.clk_domain = SrcClockDomain()
system.clk_domain.clock = '1GHz'


and

# Set the clock fequency of the system (and all of its children)
system.clk_domain = SrcClockDomain()
system.clk_domain.clock = '2GHz'


I read the first line from "stats.txt" about the sim_seconds, 1GHz takes 0.000508s, while 2GHz takes 0.000459s, which is not halved as overall system frequency is doubled. Any hints about the reason? Any other configs needed to make it as desired?

The command is: build/X86/gem5.opt configs/learning_gem5/part2/simple_memobj.py

The benchmark is: 'tests/test-progs/hello/bin/x86/linux/hello'

Thanks a lot.

[Timon Evenblij]

Hi,

I could be mistaken but I would say there are still some (perhaps default)
latency parameters in your configuration that use a time-based value
instead of a cycle-based value, or use a cycle-based value that is based on
a different clock. The main memory comes to mind first, as this often has a
default off-chip latency of 10 ns, and has its own internal clock. If you
want to investigate, I suggest to look there first.


Regards

Timon

Post by Heng Yu
Hi,
I am playing in the SE mode, with script
'GEM5_PATH/configs/learning_gem5/part2/simple_memobj.py', where I altered
# Set the clock fequency of the system (and all of its children)
system.clk_domain = SrcClockDomain()
system.clk_domain.clock = '1GHz'
and
# Set the clock fequency of the system (and all of its children)
system.clk_domain = SrcClockDomain()
system.clk_domain.clock = '2GHz'
I read the first line from "stats.txt" about the sim_seconds, 1GHz takes
0.000508s, while 2GHz takes 0.000459s, which is not halved as overall
system frequency is doubled. Any hints about the reason? Any other configs
needed to make it as desired?
The command is: build/X86/gem5.opt
configs/learning_gem5/part2/simple_memobj.py
The benchmark is: 'tests/test-progs/hello/bin/x86/linux/hello'
Thanks a lot.
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