Christoph Hellwig's on June 21, 2019 6:15 pm: > On Thu, Jun 20, 2019 at 10:21:46AM -0700, Linus Torvalds wrote: >> Hmm. Honestly, I've never seen anything like that in any kernel profiles. >> >> Compared to the problems I _do_ see (which is usually the obvious >> cache misses, and locking), it must either be in the noise or it's >> some problem specific to whatever CPU you are doing performance work >> on? >> >> I've occasionally seen pipeline hiccups in profiles, but it's usually >> been either some serious glass jaw of the core, or it's been something >> really stupid we did (or occasionally that the compiler did: one in >> particular I remember was how there was a time when gcc would narrow >> stores when it could, so if you set a bit in a word, it would do it >> with a byte store, and then when you read the whole word afterwards >> you'd get a major pipeline stall and it happened to show up in some >> really hot paths). > > I've not seen any difference in the GUP bench output here ar all. > > But I'm fine with skipping this patch for now, I have a potential > series I'm looking into that would benefit a lot from it, but we > can discusss it in that context and make sure all the other works gets in > in time. > If you can, that would be good. I don't like to object based on handwaving so I'll see if I can find any benchmarks that will give better confidence. Those old TPC-C tests were good, and there was some DB2 workload that was the reason I added gup fast in the first place. I'll do some digging. Thanks, Nick
