Christoph, On the same hardware (reboot with different kernel) I am getting _horrible_ disk I/O performance on the 5.1.1. kernel compiled on a 32-bit platform using HIGHMEM64G (PAE) to access 32GiB of physical memory. The numbers are truly terrible to copy a 16GiB file from one disk to a different one: highmem4g: 12 minutes highmem64g: 88 minutes A 733% slowdown seems really bad! Is this expected? If not, what can I do to start to debug this? This problem has existed for a long time (I have not bisected it), but I only started to care about it now that I have to make this work in production. The kernel configuration changes are below. - Matthew --- config-5.1.1.highmem4g 2019-05-11 19:54:06.000000000 -0400 +++ config-5.1.1.highmem64g 2019-05-11 20:02:26.000000000 -0400 -CONFIG_PGTABLE_LEVELS=2 +CONFIG_PGTABLE_LEVELS=3 -CONFIG_HIGHMEM4G=y -# CONFIG_HIGHMEM64G is not set +# CONFIG_HIGHMEM4G is not set +CONFIG_HIGHMEM64G=y -# CONFIG_VMSPLIT_3G_OPT is not set -# CONFIG_VMSPLIT_2G_OPT is not set +CONFIG_X86_PAE=y +# CONFIG_X86_PMEM_LEGACY is not set +CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK=y -# CONFIG_OLPC is not set +CONFIG_HAVE_ARCH_HUGE_VMAP=y +CONFIG_PHYS_ADDR_T_64BIT=y +# CONFIG_LIBNVDIMM is not set +# CONFIG_PAGE_TABLE_ISOLATION is not set +CONFIG_NEED_DMA_MAP_STATE=y +CONFIG_ARCH_DMA_ADDR_T_64BIT=y +CONFIG_SWIOTLB=y
