Demo: Two Sizes for Every Process¶

ps reports two sizes for a process: how much address space it has, and how much of it is in memory. Read both for the shell, for a browser, and for a program that prints one line.

Goal¶

Attach names to the two kinds of memory: VSZ, the virtual size, is what the process may address; RSS, the resident set size, is what it actually holds in physical memory right now.

Background¶

hello.c prints a line and waits, so that it can be measured. ps -o vsz,rss reads the two numbers from /proc/<pid>/status, in kilobytes.

Build and run¶

ps -o pid,vsz,rss,cmd -p $$
ps -o pid,vsz,rss,comm -p $(pgrep -f chrom | tail -1)
make
./hello

In a second terminal, while hello waits:

ps -o pid,vsz,rss,cmd -p $(pidof hello)

Results and explanations¶

$ ps -o pid,vsz,rss,cmd -p $$
    PID    VSZ   RSS CMD
1191993  10452  4268 bash

$ ps -o pid,vsz,rss,comm -p 32573
    PID    VSZ   RSS COMMAND
  32573 1544448496 879760 chrome

$ ps -o pid,vsz,rss,cmd -p $(pidof hello)
    PID    VSZ   RSS CMD
1195559   2752  1696 ./hello

Process	`VSZ`	`RSS`	ratio
`hello`	2.7 MB	1.7 MB	1.6
`bash`	10.2 MB	4.2 MB	2.4
`chrome`	1.44 TiB	860 MB	1,755

VSZ is never smaller than RSS: a byte that is in memory has an address, but an address need not have a byte in memory. How far apart the two are depends on the program. hello touches most of what it maps, so the ratio is small; the browser reserves a terabyte it will mostly never touch, so the ratio is in the thousands.

Even hello has 2.7 MB of address space for a one-line program. pmap shows what it is: libc, about 2 MB of it, the dynamic loader, a 132 KB heap the allocator created for stdio, a 136 KB stack, and the executable itself. Of those 2.7 MB, 1.7 MB are resident: the pages of libc the program ran through, the parts of the stack and heap it wrote, and little else.

The rest of the lecture is about the gap: what is in VSZ and not in RSS, and when it moves from one to the other.

Going further¶

grep -E 'VmSize|VmRSS|VmPeak|VmHWM' /proc/$(pidof hello)/status adds the peak of each. VmHWM, the high-water mark, is the most the process has ever had resident.
pmap -x $(pidof hello) splits RSS by area: libc's code area is 2 MB, and some 600 KB of it is resident.
top shows the same two columns as VIRT and RES, for every process, live.

References¶

man 1 ps, man 5 proc (/proc/[pid]/status)