uncached ram lkm

map uncached memory in userspace experiment.

How to use

User process requests uncached memory by calling mmap() on module's character device (see kern.log for major number). type must be MAP_SHARED and size must be a multiple of page size. See test/test.c for example doing read benchmarks on cached / uncached memory.

Demo

# make
make -C /lib/modules/4.4.0-45-generic/build/ M=/root/src/uncached_ram_lkm modules
make[1]: Entering directory `/usr/src/linux-headers-4.4.0-45-generic'
  CC [M]  /root/src/uncached_ram_lkm/uncached_ram.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /root/src/uncached_ram_lkm/uncached_ram.mod.o
  LD [M]  /root/src/uncached_ram_lkm/uncached_ram.ko
make[1]: Leaving directory `/usr/src/linux-headers-4.4.0-45-generic'

# insmod uncached_ram.ko

# tail /var/log/kern.log
Jul 27 20:42:44 localhost kernel: [115315.205744] Uncached ram module loaded
Jul 27 20:42:44 localhost kernel: [115315.205764] Created char device, major: 249

# cd test

# mknod dev c 249 0

# make
gcc -Wall -ggdb3 -std=gnu99     test.c   -o test

# ./test
usage: ./test cached   uncached_mem_dev
       ./test uncached uncached_mem_dev

# ./test cached dev
cached mem test: 104856600 reads in 1.06s                                     (sum: 0)

# ./test uncached dev
mmap()'ing dev
uncached mem test: 104856600 reads in 10.73s                                     (sum: -138595026)