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

[Ikhwan Lee <ikhwan@xxxxxxxxxxxxxxx>]

I'm using the GARNET fixed pipeline model, and it looks like the
round-robin arbitration could indeed cause different executions.
SWallocator_d::arbitrate_inports() function uses m_num_vcs to wrap
around, and since m_num_vcs = m_virtual_networks*m_vc_per_vnet,
different number of VNs could lead to different timing.

Thanks for pointing that out.

Ikhwan

On Wed, Apr 7, 2010 at 11:36 PM, Niket Agarwal <niketa@xxxxxxxxxxxxx> wrote:
> 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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>> >> >> >>
>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> > --
>> >> >> > http://www.cs.wisc.edu/~gibson [esc]:wq!
>> >> >> >
>> >> >> > _______________________________________________
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>> >> >> >
>> >> >> >
>> >> >> >
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>> >> >
>> >> >
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>> >> >
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