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.