This sounds about right. The CPU interfaces with the TLB which uses the page table walker behind the scenes. How can the TLB tell the CPU to hold off because it's busy and to try again later? It would need to do that on behalf of the table walker.
Gabe On 05/20/12 13:20, Steve Reinhardt wrote: > It occurred to me that this behavior could be related to the thread we > had before about overlapping page table walks for the same page... > Gabe and I had already agreed that if you have a TLB miss to an > address that lies within the same page (at the minimum page size) as > an ongoing walk, you really should not fire off another walk, but > that's what we currently do. Adding that suppression of redundant > walks wouldn't directly fix this problem, but it probably would > mitigate the effects. > > As far as the problem itself: even if the TLB itself has no bandwidth > limit, there should definitely be a limit on the number of concurrent > page walks, probably something in the low single digits. With a > hardware table walker, there is hardware state associated with each > ongoing walk, and the number of copies of this state to allow > concurrent walks is very limited. > > Steve > > On Sun, May 20, 2012 at 7:36 AM, Ali Saidi <sa...@umich.edu > <mailto:sa...@umich.edu>> wrote: > > Yes, there isn't any back pressure here and there > probably/maybeshould be. The reason we've got away with it before > is that normally the pressure comes from the cpu itself. The > number of outstanding translations is limited by the size of the > LSQ. That is why I find this to be strange. there is some code > that is executing and must be mis-predicting constantly to fill up > the tlb with translations and the get squashed, only to do the > exact same thing again. It seems like the second time through the > branch should be correct. The fastest way forward is probably to > add the ability to squash all pending tlb translations (except > maybe the one that is currently going on) as you through those > instructions out of the lsq on a mispredict. Make sense? > > Ali > > > > > > On May 15, 2012, at 8:48 AM, Gabe Black wrote: > > > There's a limit on the size of the TLB itself, but there may not > be a limit on the number of translations it's doing at one time. I > suspect that's an important part of the problem. > > > > Gabe > > > > > > _______________________________________________ > gem5-users mailing list > gem5-users@gem5.org <mailto:gem5-users@gem5.org> > http://m5sim.org/cgi-bin/mailman/listinfo/gem5-users > > > > _______________________________________________ > gem5-users mailing list > gem5-users@gem5.org > http://m5sim.org/cgi-bin/mailman/listinfo/gem5-users
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