geom/zero: Add support for unmapped I/O
This patch adds support for unmapped I/O to gzero(4).
Let's consider the following script to illustrate the change in
gzero(4)'s behavior:
dd="dd if=/dev/gzero of=/dev/null bs=512 count=100000"
dtrace -q -c "$dd" -n '
fbt::pmap_qenter:entry,
fbt::uiomove_fromphys:entry,
fbt::memset:entry
/execname == "dd"/
{
@[probefunc] = count();
}
'Let's run that script 4 times:
==> 1: unmapped I/O not supported (fallback to mapped I/O), kern.geom.zero.clear=1 51200000 bytes transferred in 1.795809 secs (28510829 bytes/sec) pmap_qenter 100000 memset 400011 ==> 2: unmapped I/O not supported (fallback to mapped I/O), kern.geom.zero.clear=0 51200000 bytes transferred in 0.701079 secs (73030337 bytes/sec) memset 300011 ==> 3: unmapped I/O supported, kern.geom.zero.clear=1 51200000 bytes transferred in 0.771680 secs (66348750 bytes/sec) uiomove_fromphys 100000 memset 300011 ==> 4: unmapped I/O supported, kern.geom.zero.clear=0 51200000 bytes transferred in 0.621303 secs (82407407 bytes/sec) memset 300011
If kern.geom.zero.clear=0, then nothing really changes as no copying takes
place. Otherwise, we see by adding unmapped I/O support we avoid calls to
pmap_qenter(), which was called by GEOM to turn unmapped I/O requests into
mapped ones before passing them for processing to gzero(4).
Reviewed by: bnovkov, markj
Approved by: bnovkov (mentor), markj (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D52998
(cherry picked from commit d01183fddf37b15da7ab3d69039e7759ae144451)