What kind of benchmark are you running? And when you run spec which input size? I could imagine this happening if there is positive aliasing, and by increasing the size you reduce the aliasing, thus increasing the warm up time of the predictor. Especially if you have small benchmarks. On Jan 16, 2014 10:25 PM, "Milad Mohammadi" <[email protected]> wrote:
Thanks for the quick response! What I do is I comment out the block of code in bpred_impl.hh that checks for if (!isCondCtrl()) and let unconditional branches be looked up in the local predictor. Once this code is commented, I run several simulations each with a different BP size: 32, 256, 1024, 2048, 4096 entries In the BP (the rest of the setup is the same as the original model) and what I observed is a monotonic increase in the number of mispredictions. I have spend several days debugging the code and as far as I can see, things are setup fine... So, I'm guessing this is a bug that quite under the hood. Let me add that I have run several spec benchmarks and they all exhibit the same behavior. I'm using the version of gem5 from summer 2012. Also, I believe this behavior is wrong because as the size of the predictor increases aliasing probability drops and he number of mispredictions ought to drop (unless a corner case happens....) On Thursday, January 16, 2014, Anthony Gutierrez <[email protected]> wrote: > I'm not quite sure exactly what it is you're doing, but if I am following > correctly it doesn't seem like a bug. If you are allowing unconditional > branches to pollute the predictor's history tables with an "always taken" > prediction for that particular PC, then the behavior describing could be > reasonable. > > > Anthony Gutierrez > http://web.eecs.umich.edu/~atgutier > > > On Thu, Jan 16, 2014 at 9:01 PM, Milad Mohammadi <[email protected]>wrote: > >> Hi, >> >> The gem5 local branch predictor exhibits a larger number of mispredictions >> as you increase the size of the predictor. This happens when I allow >> "unconditional" branches to be looked up in the predictor. >> >> Please advise what is the reason behind this bug. >> The correct behavior is regardless what type of instructions are looked up >> in the BP as the BP size increases, the accuracy drops. >> >> Thanks! >> Milad >> _______________________________________________ >> gem5-dev mailing list >> [email protected] >> http://m5sim.org/mailman/listinfo/gem5-dev >> > > _______________________________________________ gem5-users mailing list [email protected] http://m5sim.org/cgi-bin/mailman/listinfo/gem5-users
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