Some things about getting useful output from
No version of
time has ever had what you could call a genuinely useful
default output format, at least not for benchmarking and testing. The
POSIX (and System V) standard format has all of the information you want
in the right format (the times in simple seconds), but spreads it across
multiple lines; the BSD format used by
csh puts the times in 'human
readable' format of hours, minutes, and seconds, and includes a bunch
of additional information (much of which is inaccurate, useless, or
outright made up by the kernel).
Fortunately, GNU time lets you set a different format. Since I keep looking this up, here is what I want to use:
time -f '%e real %U user %S kernel' ....
I want the times in pure seconds, not in 'human readable' converted form, because I am almost invariably performing math on them in order to find out the information that I'm really interested in.
(Perhaps somewhere there is a convenient Python module for doing this
with human readable numbers, but frankly
bc works fine with seconds so
my motivation to go looking is low.)
time program has no format specifier option, but defaults
to essentially this format. Solaris's
time program also has no format
specifier option and defaults to a 'human readable' multi-line format;
the best you can do is make it use the POSIX format with '
On basically all systems,
tcsh have their own
which uses the BSD
csh format or some variant of it (and can be
customized to some extent).
Now, let's talk about why I say that
time makes up some of its
time generally gets its numbers from
technically it gets them from
wait4(), which returns the same
information). However, various versions of
document that some information is plain not returned or is not what you
think it is. The Linux manpage for
getrusage() says, for example:
[...] Not all fields are meaningful under Linux. In linux 2.4 only the fields
ru_majfltare maintained. Since Linux 2.6,
ru_nivcsware also maintained.
time determines the wall clock time itself.)
This corresponds to
time's user and system time, major and minor page
faults, and voluntary and involuntary time switches (%w and %c, not
printed by default), and it assumes that the Linux kernel does a good
job of keeping track of this information, which I would be dubious of.
The kernel has good intentions, but doing completely accurate accounting
can be hard and is not necessarily a high priority.
(Current Linux 2.6 kernels seems to also produce numbers for IO input and output, provided that you have the right kernel configuration options turned on. Again, the trustworthiness of those numbers is unknown.)
Currently, FreeBSD 8.0 says:
ru_oublockaccount only for real I/O; data supplied by the caching mechanism is charged only to the first process to read or write the data.
I'm not going to quote from the Solaris 10
getrusage() manpage, but it has a
similar long list of caveats and cautions.
Now, it's possible that
time will return real and useful
information for things you are interested in (beyond the basic time
information). But before you rely on its numbers for performance
measurement, you certainly need to check to make sure that they
are accurate and useful. This should include not just reading the
getrusage() manpage for your OS, but actually constructing synthetic
programs that perform known operations and making sure that
reports accurate information for what they do.