Re: [Gems-users] irregular statistics due to the number of messages injected into the network by the coherence protocol

[Alberto Ros Bardisa <a.ros@xxxxxxxxxxx>]

Hi Jesus,

Regarding the drop in the number of messages when going to latencies of 
128 cycles, my opinion is that maybe the number of cache misses is 
reduced, particularly, the number of coherence misses. Being the latency 
longer, caches could hold contended blocks more time, thus reducing miss 
rate. You could check the L1 mis rate and, in particular, coherence 
misses. However, latencies are longer and that's why execution time is 
worse.
Alberto.

Jesus Camacho Villanueva escribió:
> Hi everybody!
>
> We are simulating several applications in a sarek machine and using the
> MESI_CMP_directory protocol.
>
> At first, we simulated barnes and raytrace applications with different
> topologies and we observed that the average (from 10 seeds) amount of
> messages that were injected into the network differed in some cases more
> than 50%!!! depending on the topology used.
>
> Recently, we have run an FFT application with a basic network model that
> introduces a constant message delay (that means we simulate the same
> latency for each message going from any node to any other node) and we
> have obtained the following results:
>
> Constant latency (cycles) - execution time (cycles) - number of messages
> - page_reclaims
> 1 - 3361858,8 - 830518 - 1018051,1
> 8 - 4346059,2 - 945225,1 - 1132667,2
> 16 - 5685143,7 - 958207,5 - 1145672,2
> 32 - 8410379,9 - 977663,1 - 1165158,3
> 64 - 14785582,6 - 1109518,3 - 1297078,7
> 128 - 20265856,9 - 594415,7 - 781911,1
>
> 10 different seeds have been used and average results plotted. As you
> see, the execution time goes always up which is normal (the network
> delay increases). We figure out the slight increase in the number of
> messages and pages accessed is normal since the application is taking
> more time. However, the severe drop when going from 64 cycles to 128
> cycles was totally unexpected and we didn't find any reasonable
> explanation, so we figure out there should be something wrong. The funny
> thing is that the application finishes with half the pages claimed.
>
> Anyone knows what it could be happening?
>
> Any help will be really appreciated, since we are blocked with this issue!
>
> Thanks a lot,
>
> Jesús.
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/* Alberto Ros Bardisa                                             */
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