Bug 13206

The time command has an ability to show the maximum resident set size in rusage struct. For example, you can see it with "-v" option.

$ busybox time --version
time: unrecognized option '--version'
BusyBox v1.27.2 (Ubuntu 1:1.27.2-2ubuntu3.2) multi-call binary.
$ busybox time -v echo 
	Command being timed: "echo"
	User time (seconds): 0.00
	System time (seconds): 0.00
	Percent of CPU this job got: ?%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0m 0.00s
	Average shared text size (kbytes): 0
	Average unshared data size (kbytes): 0
	Average stack size (kbytes): 0
	Average total size (kbytes): 0
	Maximum resident set size (kbytes): 7936
	Average resident set size (kbytes): 0
	Major (requiring I/O) page faults: 0
	Minor (reclaiming a frame) page faults: 62
	Voluntary context switches: 1
	Involuntary context switches: 1
	Swaps: 0
	File system inputs: 0
	File system outputs: 0
	Socket messages sent: 0
	Socket messages received: 0
	Signals delivered: 0
	Page size (bytes): 4096
	Exit status: 0

I guess busybox's time command is based on gnu time version 1.7. This version of time command expects ru.ru_maxrss is page size instead of kilo bytes.

> case 'M':   /* Maximum resident set size.  */
>     printf("%lu", ptok(pagesize, (UL) resp->ru.ru_maxrss));
>     break;

However, this assumption is wrong for some operating systems. In my environment (Ubuntu 18.04), busybox's time command shows a 4 times larger value than the correct value.

> While other entries are documented as pages that entry is without units attached to the comment. That is undoubtedly the source of the error. Pages were assumed when it was really kbytes. Looking at the libc documentation we see that it is not ambiguous:
> 
> https://groups.google.com/g/gnu.utils.help/c/u1MOsHL4bhg/m/ewaNE5uxLxoJ?pli=1

Then, gnu time 1.8 fixed in this commit. This fix introduced a configure option which unit is used for ru.ru_maxrss. 
https://git.savannah.gnu.org/cgit/time.git/commit/?id=fe24b572fca28dc9922096d21b665736dc477534

I think busybox's time command also need to fix what gnu time 1.8 fixed.