Re: [Gems-users] What happens if NUMBER_OF_VIRTUAL_NETWORKS is greater than needed?

[Niket Agarwal <niketa@xxxxxxxxxxxxx>]

What interconnect model you are using? In GARNET, most arbitrations across different VNs are independent. So, having extra unused VNs should not matter. Having said that, I can imagine round-robin arbitrations inside the router taking separate decisions with different number of VNs. This could lead to different order of packet delivery and hence different executions.

-Niket

On 04/08/2010 12:31 AM, Dan Gibson wrote:

Well, actually the same argument applies. As far as I understand the situation, your two runs (VN=4 and VN=5) could have had different g_RANDOM_SEEDs, which could account for the difference in performance. Admittedly, I'm no expert on Ruby's interconnect, so I don't know what the actual effect of extra VCs would be... but it seems like the kind of thing that would have an effect.

Regards,
Dan

On Wed, Apr 7, 2010 at 8:57 PM, Ikhwan Lee <ikhwan@xxxxxxxxxxxxxxx> wrote:

Dan,
I was not trying to compare the performance of VN=4 and VN=5. I was
rather concerned about the integrity of the simulation results since I
could not understand why they weren't exactly the same. But I do
appreciate your advice. Thanks,
mos
Ikhwan

On Tue, Apr 6, 2010 at 8:17 AM, Dan Gibson <degibson@xxxxxxxx> wrote:
> Ikhwan,
> Let me put it another way. In multiprocessor simulations, no two runs are
> different in a /statistically significant way/. Only when two configurations
> (e.g., VN=4, VN=5) are run each MANY times, and averages taken, with nice,
> tight 95% confidence intervals, can one say with confidence that X is faster
> than Y.
>
> I.e., try VN=4 and VN=5 each with many different RANDOM_SEEDs.
>
> Regards,
> Dan
>
> See also:
> "Variability in Architectural Simulations of Multi-threaded Workloads", by
> Wisconsin's own Alaa Alameldeen and David A. Wood.
> LINK: http://www.cs.wisc.edu/~alaa/papers/hpca03_variability.pdf
>
>
> On Mon, Apr 5, 2010 at 5:07 PM, Ikhwan Lee <ikhwan@xxxxxxxxxxxxxxx> wrote:
>>
>> Thanks Dan. I was running MOESI_SMP_directory using a fixed
>> g_RANDOM_SEED for all simulations. I am still not sure why results are
>> different for having an extra VNET. I'll keep the list posted if I
>> figure out why.
>>
>> Ikhwan
>>
>> On Tue, Apr 6, 2010 at 7:57 AM, Dan Gibson <degibson@xxxxxxxx> wrote:
>> > It depends.
>> > 1. If you're not using a protocol ending in _m, then your protocol
>> > doesn't
>> > use the MemoryController module. If that is the case, random noise (aka
>> > rand() % 5) is simply added to MAIN_MEMORY_LATENCY_MINUS_2 to generate
>> > the
>> > 'actual' memory latency.
>> > 2. If your protocol ends in _m, then you are using the MemoryController
>> > model, which has some built-in features to add random delays, when
>> > activated. Check out MemoryController.[Ch] (specifically, the comments
>> > at
>> > the start) to see how that works.
>> >
>> > In either case, for a fixed RANDOM_SEED, you should get precisely the
>> > same
>> > results.
>> >
>> > Regards,
>> > Dan
>> >
>> > On Mon, Apr 5, 2010 at 4:43 PM, Ikhwan Lee <ikhwan@xxxxxxxxxxxxxxx>
>> > wrote:
>> >>
>> >> How is a simulation randomized? I've been getting exactly the same
>> >> results for each run with same simulation parameters.
>> >>
>> >> Thanks,
>> >> Ikhwan
>> >>
>> >> On Sat, Apr 3, 2010 at 12:24 PM, Dan Gibson <degibson@xxxxxxxx> wrote:
>> >> > Average many runs of each (e.g., 8 runs w/ VNs = 4, 8 runs w/ VNs =
>> >> > 5)
>> >> > and
>> >> > see if there is a statistically significant difference or not. Since
>> >> > the
>> >> > difference in runtime is pretty small, this could be due only to
>> >> > difference
>> >> > in RANDOM_SEED.
>> >> >
>> >> > That said, I don't actually know how Ruby might react to having
>> >> > 'extra'
>> >> > VNs.
>> >> >
>> >> > Regards,
>> >> > Dan
>> >> >
>> >> > On Fri, Apr 2, 2010 at 4:52 PM, Ikhwan Lee <ikhwan@xxxxxxxxxxxxxxx>
>> >> > wrote:
>> >> >>
>> >> >> The MOESI_SMP_directory protocol needs 4 virtual networks; however,
>> >> >> it
>> >> >> should also run fine with 5 virtual networks since the 5th virtual
>> >> >> network will never be used by the protocol. The question is, when I
>> >> >> compare the results of two simulations (one with 4 and one with 5
>> >> >> virtual networks), I get something like below.
>> >> >>
>> >> >> 153c153
>> >> >> < NUMBER_OF_VIRTUAL_NETWORKS: 4
>> >> >> ---
>> >> >> < NUMBER_OF_VIRTUAL_NETWORKS: 5
>> >> >> 1457c1458
>> >> >> < Ruby_current_time: 25741756
>> >> >> ---
>> >> >> > Ruby_current_time: 27894100
>> >> >> 1459c1460
>> >> >> < Ruby_cycles: 25741755
>> >> >> ---
>> >> >> > Ruby_cycles: 27894099
>> >> >> 1461,1468c1462,1469
>> >> >> < mbytes_resident: 480.223
>> >> >> < mbytes_total: 508.523
>> >> >> < resident_ratio: 0.944347
>> >> >> <
>> >> >> < Total_misses: 2736589
>> >> >> < total_misses: 2736589 [ 221437 184551 181803 181854 165005 165866
>> >> >> 172800 177348 162455 164810 168559 167311 146275 158797 158483
>> >> >> 159235
>> >> >> ]
>> >> >> < user_misses: 199059 [ 6640 12908 16388 12543 9998 14914 11967
>> >> >> 11834
>> >> >> 17170 11744 11685 12695 13389 11501 12351 11332 ]
>> >> >> < supervisor_misses: 2537530 [ 214797 171643 165415 169311 155007
>> >> >> 150952 160833 165514 145285 153066 156874 154616 132886 147296
>> >> >> 146132
>> >> >> 147903 ]
>> >> >> ---
>> >> >> > mbytes_resident: 177.781
>> >> >> > mbytes_total: 498.574
>> >> >> > resident_ratio: 0.356579
>> >> >> >
>> >> >> > Total_misses: 2976004
>> >> >> > total_misses: 2976004 [ 231381 201401 206063 197190 177795 182883
>> >> >> > 187239
>> >> >> > 196167 175355 180539 183532 179732 157077 173355 171734 174561 ]
>> >> >> > user_misses: 200169 [ 12482 12593 12510 11561 13957 9487 11581
>> >> >> > 12406
>> >> >> > 17732 13232 11811 11721 12437 12137 12108 12414 ]
>> >> >> > supervisor_misses: 2775835 [ 218899 188808 193553 185629 163838
>> >> >> > 173396
>> >> >> > 175658 183761 157623 167307 171721 168011 144640 161218 159626
>> >> >> > 162147
>> >> >> > ]
>> >> >> --- omitted the rest
>> >> >>
>> >> >> The differences are not too big, but still quite strange since I
>> >> >> expected them to be exactly the same. Any idea on what is happening?
>> >> >> I'm using Garnet fixed pipeline network.
>> >> >>
>> >> >> Thanks,
>> >> >> Ikhwan
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