What's the unit of `ru_maxrss` on Linux?

Question

This is from man getrusage

struct rusage {
    struct timeval ru_utime; /* user time used */
    struct timeval ru_stime; /* system time used */
    long   ru_maxrss;        /* maximum resident set size */
    long   ru_ixrss;         /* integral shared memory size */
    long   ru_idrss;         /* integral unshared data size */
    long   ru_isrss;         /* integral unshared stack size */
    long   ru_minflt;        /* page reclaims */
    long   ru_majflt;        /* page faults */
    long   ru_nswap;         /* swaps */
    long   ru_inblock;       /* block input operations */
    long   ru_oublock;       /* block output operations */
    long   ru_msgsnd;        /* messages sent */
    long   ru_msgrcv;        /* messages received */
    long   ru_nsignals;      /* signals received */
    long   ru_nvcsw;         /* voluntary context switches */
    long   ru_nivcsw;        /* involuntary context switches */
};

However it's not specified what's the unit.

I saw FreeBSD's documentation which says it's in kilobytes, but I'm not sure about what unit it is on Linux.

Acromion

Answer 1

It's not a standard field for the rusage structure so POSIX doesn't mandate anything about it. But on Linux

ru_maxrss (since Linux 2.6.32)

This is the maximum resident set size used (in kilobytes). For RUSAGE_CHILDREN, this is the resident set size of the largest child, not the maximum resident set size of the process tree.

Asdic

Actually, they are kibibytes (1024 bytes), and not kilobytes (1000 bytes) – Lariat

Answer 2

The man page says:

ru_maxrss (since Linux 2.6.32)

This is the maximum resident set size used (in kilobytes). For RUSAGE_CHILDREN, this is the resident set size of the largest child, not the maximum resident set size of the process tree.

So, it's expressed in kilobytes, just like in BSD.

Hamlen

Actually, they are kibibytes (1024 bytes), and not kilobytes (1000 bytes) – Lariat

@Lariat in the context of the Linux kernel and the GNU C Library, "kilobyte" means 1024 bytes unless otherwise noted. Traditionally, it has been accepted that "kilobyte" means either 1024 or 1000 bytes depending on the context, and in these contexts it usually means 1024. The proposal that "kilobyte" always mean 1000, and 1024 be called "kibibytes" instead, has not been generally accepted by these development communities – Spinule

Linux predates the word kibibyte. But now we have it, and so all of the documentation should be updated. – Lariat