Do you still get the slowdown if you mmap /dev/zero rather than your driver?
Forwarding the responce:
Transfers from /dev/zero -> /dev/shm are unreasonably high (e.g. 128.000 MB in 0.000028 sec), which suggests that there isn't really any I/O going on here.
Transfers from /dev/zero to disk are approximately 2x faster, but still slow. Instead of around 20MB/s it looks like around 40 MB/s.
John
