windowsaio: add best effort cache invalidation

windowsaio: add best effort cache invalidation

https://stackoverflow.com/questions/478340/clear-file-cache-to-repeat-performance-testing/7113153#7113153
mentions that opening a file without buffering on Windows has the side
effect of flushing cached parts of the file out of RAM. Use this side
effect to do cache invalidation on non-direct jobs that are using the
windowsaio engine.

This change may make default bandwidth speeds on Windows look lower
compared to older versions of fio but this matches the behaviour of fio
on other platforms with invalidation (such as Linux) because we are
trying to avoid measuring cache reuse (unless invalidate=0 is set).

Note that cache invalidation will fail to happen if the file is also
open elsewhere. Unfortunately, we can't check whether we have exclusive
access without introducing a race where other jobs/programs fail during
open because we temporarily have exclusive access to the file.

Invalidation is done as a call from within fio_windowsaio_open_file
because invalidation fails with multiple file handles (see above) and
it's simpler than trying to use ReOpenFile (which is only available from
Windows 2003/Windows Vista onwards).

The impact of invalidation is demonstrated by the bandwidths achieved by
the following jobs running on a hard disk of an otherwise idle system
with 4GBytes of RAM:
./fio --stonewall --thread --size=128M --filename=fio.tmp --iodepth=64 \
  --bs=128k --invalidate=0 --direct=0 \
  --name=create --rw=write \
  --name=cached --rw=read --loops=2 \
  --name=invalidated --rw=read --loops=2 --invalidate=1

[...]
cached: (groupid=1, jobs=1): err= 0: pid=3888: Sat Oct 7 08:05:39 2017
   read: IOPS=11.7k, BW=1463MiB/s (1534MB/s)(256MiB/175msec)
[...]
invalidated: (groupid=2, jobs=1): err= 0: pid=828: Sat Oct 7 08:05:39 2017
   read: IOPS=2089, BW=261MiB/s (274MB/s)(256MiB/980msec)

v2:
- Move invalidation into a helper function.
- Add more detail and rationale to the commit message.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>