I'm extremely sorry if I worded my question incorrectly. I'm actually
trying to introduce a delay whenever a read/write request happens in the
main memory. For example, in a memory write, the data would only be flagged
as dirty after a 10ns delay.
Regardless, thanks a lot for the response!
On Sat, Jun 10, 2023 at 12:28 PM Eliot Moss <m...@cs.umass.edu> wrote:
> On 6/10/2023 11:12 AM, Vincent Abraham via gem5-users wrote:
> > Hi everyone,
> > I'm trying to model additional latencies in the main memory while
> performing write/read operations.
> > Could anyone tell me how I could go about doing it?
>
> Of course the dram module has a gazillion timing and energy parameters that
> you can simply look up in your config.ini file, if that's what you mean.
>
> But I suspect you want to know something like the distribution of access
> times. You can see comm_monitor for statistics examples (and the memory
> controller already has a lot as well), but it might go roughly like this.
>
> First, set up a map in the controller module along these lines:
>
> hash_map<PacketPtr, Tick> arrivalTime;
>
> When a packet arrives at the controller, put it into the map like this:
>
> arrivalTime.emplace(pkt, curTick());
>
> Later, when the packet has finished processing, you can find out how long
> it
> took by code like this:
>
> auto it = arrivalTime.find(pkt);
> assert(it != arrivalTime.end); // it really should be there
> Tick arrival = it->second;
> Tick latency = curTick() - arrival;
> arrivalTime.erase(it);
>
> The other part is recording statistics. To get a histogram over all
> packets,
> declare a stat like this as a member of the memory controller's class:
>
> Stats::Histogram pktLatencies;
>
> In the controller module's regStats function, add this:
>
> pktLatencies
> .init(20) // or whatever number of buckets you want
> .name(name() + ".pkt_latencies")
> .desc("Histogram of packet latencies")
> .flags(cdf | dist | nozero); // I like cdf, but pdf can be good too
>
> Of course you don't have to use a histogram, and you don't have to use just
> one. For example, you could have one for reads and one for writes, and
> enter
> packets conditionalized on isRead() and isWrite().
>
> The one piece I did not mention yet is adding a sample to the histogram.
> When
> a packet finishes and you know its latency, just do:
>
> arrivalTimes.sample(latency);
>
> The magic of stats will do the rest.
>
> Cheers - Eliot Moss
>
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