On 12/05/2015 12:19, Denis V. Lunev wrote: > > > hades /vol $ strace -f -e pwrite -e raw=write,pwrite qemu-io -n -c > "write -P 0x11 0 64M" ./1.img > Process 19326 attached > [pid 19326] pwrite(0x6, 0x7fac07fff200, 0x4000000, 0x50000) = 0x4000000 > <---- 1 GB Write from userspace
FWIW this is 64 MB (as expected). > wrote 67108864/67108864 bytes at offset 0 > 64 MiB, 1 ops; 0.2964 sec (215.863 MiB/sec and 3.3729 ops/sec) > [pid 19326] +++ exited with 0 +++ > +++ exited with 0 +++ > hades /vol $ > 9,0 1 266 74.030359772 19326 Q WS 473095 + 1016 [(null)] > 9,0 1 267 74.030361546 19326 Q WS 474111 + 8 [(null)] > 9,0 1 268 74.030395522 19326 Q WS 474119 + 1016 [(null)] > 9,0 1 269 74.030397509 19326 Q WS 475135 + 8 [(null)] > > This means, yes, kernel is INEFFECTIVE performing direct IO with > not aligned address. For example, without direct IO the pattern is > much better. I think this means that the kernel is DMAing at most 128 pages at a time. If the buffer is misaligned, you need 129 pages and the kernel then splits the request into a 128 page and a 1 page part. This looks like a hardware limit, and the kernel probably cannot really do anything about it because we requested O_DIRECT. So your patch makes sense. Paolo
