Fault in the buffer prior to writing to workaround poor performance due
to interaction with kernel fs deadlock avoidance code. See the comment
prior to vn_io_fault_doio() in sys/kern/vfs_vnops.c for details of the
issue.
On my stable/10 desktop with a 16MB obj.o and "ar r out.a obj.o" I see
the following run times (seconds):
x ar.r284891
+ ar.patched
+----------------------------------------------------------------------+
|+ |
|+ x|
|+ xx|
|A |A|
+----------------------------------------------------------------------+
N Min Max Median Avg Stddev
x 3 1.307 1.321 1.315 1.3143333 0.0070237692
+ 3 0.020 0.023 0.022 0.021666667 0.0015275252
Difference at 95.0% confidence
-1.29267 +/- 0.0115203
-98.3515% +/- 0.876513%
(Student's t, pooled s = 0.00508265)
Thanks to kib for diagnosing and explaining the issue and suggesting
the workaround.
Reviewed by: eadler, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D2933
01715d8cd1