> > One corner case needs to mention is that the PEBS hardware doesn't > > deal well with collisions, when PEBS events happen near to each other. > > The records for the events can be collapsed into a single one, and > > it's not possible to reconstruct all events that caused the PEBS > > record, However in practice collisions are extremely rare, as long as > > different events are used. The periods are typically very large, so > > any collision is unlikely. When collision happens, we drop the PEBS > > record. > > > > Here are some numbers about collisions. > > Four frequently occurring events > > (cycles:p,instructions:p,branches:p,mem-stores:p) are tested > > > > Test events which are sampled together collision rate > > cycles:p,instructions:p 0.25% > > cycles:p,instructions:p,branches:p 0.30% > > cycles:p,instructions:p,branches:p,mem-stores:p 0.35% > > > > cycles:p,cycles:p 98.52% > > > > collisions are extremely rare as long as different events are used. > > The only way you can get a lot of collision is when you count the same > > thing multiple times. But it is not a useful configuration. > > This fails to mention the other problem the status field has. You also did > not specify what exact condition you counted as a collision. > > The PEBS status field is a copy of the GLOBAL_STATUS MSR at assist time, > this means that: > > - its possible (and harmless) for the status field to contain set bits > for !PEBS events -- the proposed code is buggy here. I will fix it. > - its possible to have multiple PEBS bits set even though the event > really only was for a single event -- if you count everything with > multiple PEBS bits set as a collision you're counting wrong. >
In what situation multiple PEBS bits was set for a single event? Could you please give me an example? Thanks, Kan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
