> > > > Hi, > > > > > > > > The qemu process will stuck when hot-add large size memory to > > > > the virtual machine with a device passtrhough. > > > > We found it is too slow to pin and map pages in vfio_dma_do_map. > > > > Is there any method to improve this process? > > > > > > At what size do you start to see problems? The time to map a > > > section of memory should be directly proportional to the size. As > > > the size is increased, it will take longer, but I don't know why > > > you'd reach a point of not making forward progress. Is it actually > > > stuck or is it just taking longer than you want? Using hugepages > > > can certainly help, we still need to pin each PAGE_SIZE page within > > > the hugepage, but we'll have larger contiguous regions and therefore > > > call iommu_map() less frequently. Please share more data. Thanks, > > > > > > Alex > > It just take longer time, instead of actually stuck. > > We found that the problem exist when we hot-added 16G memory. And it > > will consume tens of minutes when we hot-added 1T memory. > > Is the stall adding 1TB roughly 64 times the stall adding 16GB or do we > have some inflection in the size vs time curve? There is a cost to > pinning an mapping through the IOMMU, perhaps we can improve that, but I > don't see how we can eliminate it or how it wouldn't be at least linear > compared to the size of memory added without moving to a page request > model, which hardly any hardware currently supports. A workaround might > be to incrementally add memory in smaller chunks which generate a less > noticeable stall. Thanks, > > Alex It took about 1 minute to add 16GB and about 40 minutes to add 1TB.
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