On 11/4/21 23:45, David Matlack wrote: [...] > > The last alternative is to perform dirty tracking at a 2M granularity. > This would reduce the amount of splitting work required by 512x, > making the current approach of splitting on fault less impactful to > customer performance. We are in the early stages of investigating 2M > dirty tracking internally but it will be a while before it is proven > and ready for production. Furthermore there may be scenarios where > dirty tracking at 4K would be preferable to reduce the amount of > memory that needs to be demand-faulted during precopy. I'm curious how you're going about evaluating this, as I've experimented with 2M dirty tracking in the past, in a continuous checkpointing context however. I suspect it's very sensitive to the workload. If the coarser granularity leads to more memory being considered dirty, the length of pre-copy rounds increases, giving the workload more time to dirty even more memory. Ideally large pages would be used only for regions that won't be dirty or regions that would also be pretty much completely dirty when tracking at 4K. But deciding the granularity adaptively is hard, doing 2M tracking instead of 4K robs you of the very information you'd need to judge that.
