Il 11/04/2013 15:24, Michael R. Hines ha scritto: > That's very accurate. Zero page scanning *after* the bulk phase > is not very helpful in general. > > Are we proposing to skip is_dup_page() after the bulk phase > has finished?
No, I'm saying that is_dup_page() should not be a problem. I'm saying it should only loop a lot during the bulk phase. The only effect I can imagine after the bulk phase is one cache miss. Perhaps the stress-test you're using does not reproduce realistic conditions with respect to zero pages. Peter Lieven benchmarked real guests, both Linux and Windows, and confirmed the theory that I mentioned upthread. Almost all non-zero pages are detected within the first few words, and almost all zero pages come from the bulk phase. Considering that one cache miss, RDMA is indeed different here. TCP would have this cache miss later anyway, RDMA does not. Let's say 300 cycles/miss; at 2.5 GHz that is 300/2500 microseconds, i.e 0.12 microseconds per page. This would say that we can run is_dup_page on 30 GB worth of nonzero pages every second or more. Ok, the estimate is quite generous in many ways, but is_dup_page() is only a bottleneck if it can do less than 5 GB/s. Paolo
