Re: [Gems-users] Cache Parameters

[Mike Marty <mikem@xxxxxxxxxxx>]

Vince,

I recommend using Ruby to simulate the cache hierarchy.  But the latencies
seen by the processor will ultimately depend on the network, cache
coherence protocol, and their various parameters.

Many Ruby parameters are listed in ruby/config/rubydefaults.config

For example you can set the MEMORY_LATENCY parameter, but this does not
take into account network delay.

L1 hit latency is 1-cycle by default.  To increase this to 2-cycles, you
will need to remove the "fast path" hits.  What this means is that our
"Sequencer" (or processor/Simics interface) will check to see if it has
permission to the cache block before actually making a request to Ruby.
One reason we do this is to decrease simulation time, however the problem
is that all hits are the 1-cycle.  There is a flag
ruby/config/rubydefaults.config to turn this off and force all requests to
enter Ruby such that you can vary the latency however you wish.

To tune the parameters to match what your end-to-end latency goals are, I
recommend using the debug tracer (see the Debugging README on the
documentation website) to actually see how long various things typically
take.  Using the (random or deterministic) tester facilities would be
useful here as well.

--Mike



 > Hello,
>
> I'm trying to set Opal up to simulate a cache heiarchy with the following
> parameters:
>
>   Cache-to-cache latency - 400 cycles
>   L1 hit latency - 2 cycles
>   L2 hit latency - 24 cycles
>   Memory access latency - 460 cycles
>
> The latter two seem obvious:
>   opal0.setparam L2_LATENCY 24
>   opal0.setparam MEMORY_DRAM_LATENCY 460
>
> The L1 latency is more problematic... the options are a bit similar.
>
> Is the
>   opal0.setparam L1_FILL_BUS_CYCLES
> option what I want?
>
> What should L2_FILL_BUS_CYCLES be in this case?
>
> I've been going through the default config files and also the code itself
> but am a bit confused by what's going on...
>
> Thanks,
>
> Vince
>
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