22e6582974
If a system's time is adjusted backwards, then elapsed
time could appear as negative. This yielded a negative
%CPU value. Alternately if zeros were suppressed ('0')
the result was a blank %CPU column. In both cases that
distortion would last for one display cycle or until a
user forced a display refresh via some keyboard input.
The original recommendation was trading gettimeofday()
for clock_gettime() using CLOCK_MONOTONIC. But on some
systems that might not be possible, forcing the use of
CLOCK_REALTIME instead. Not only would that complicate
the build system, but it may leave us with minus %CPU.
Another approach was to ensure that elapsed time could
never be negative. Of course, this produced distortion
of %CPU values but it would be proportionally correct.
This wasn't dissimilar to a distortion already present
should the time be adjusted forward or backward within
any 'remaining' top delay intervals. These aberrations
would be avoided with clock_gettime & CLOCK_MONOTONIC,
but that is a less than ideal solution as noted above.
This final solution, which originated down under, will
simply rely on the /proc/uptime seconds, which will be
immune to *any* tampering with the system clock. Thus,
we now have a fix for the distortion we didn't know we
suffered plus a negative %CPU that began this odyssey.
Thanks to:
sk.alvin.x@gmail.com, for the original effort
jcapik@redhat.com, for a heads up on CLOCK_MONOTONIC
csmall-procps@enc.com.au, for the best suggestion of all
Reference(s):
. original post/patch
http://www.freelists.org/post/procps/PATCH-top-use-clock-gettime-instead-of-gettimeofday
. heads up on CLOCK_MONOTONIC
http://www.freelists.org/post/procps/PATCH-top-use-clock-gettime-instead-of-gettimeofday,2
. the final solution
http://www.freelists.org/post/procps/PATCH-top-use-clock-gettime-instead-of-gettimeofday,11
Signed-off-by: Jim Warner <james.warner@comcast.net>
COMPATIBILITY
This code is intended for use with Linux 2.6.xx, 3.x and
hopefully all future kernels.
INSTALLATION
If you are using git version of the project you need extra step.
./autogen.sh
After that, and everyone using .tar.xz version of procps-ng, can
do normal build. Read './configure --help' to select options for
your needs.
./configure
make
make install
If you have DejaGNU installed you can run optional test suite.
make check
HOW TO CONTRIBUTE
See Documentation/BUGS file.
PACKAGING
If you are a downstream maintainer (packager) for a Linux
distribution, please avoid causing troubles. This section
applies to you.
Avoid maintaining distribution specific patches. Send your
patches to upstream, where they are at least reviewed, if not
included.
Please forward bug reports. If your bug database is public and
busy enough to bother with, please make this known. Follow
Debian's lead in making the bug database easy to comment on via
email without need for an account.
For normal packages, ensure that you do not add debugging flags
to the CFLAGS variable.
UPSTREAM & BUG REPORTS
procps-ng <procps@freelists.org>