getrusage quirks (on Linux, ru_maxrss is the max for the process, regardless of the exec after a fork)

.gitignore

*.o

show-maxrss
bloat-fe

bloat-fe.c

#include <stdlib.h>
#include <sys/resource.h>

#include "runmaxrss.h"
#include "show-maxrss-lib.h"

#define CHONK (1024*1024*1024) // 1 GB

int main() {
  showmaxrss();
  show(RUSAGE_SELF, "bloat-fe after malloc");
  char *m = (char*)malloc(CHONK);
  for (int i = 0; i < CHONK; i += 1024*1024) {
    m[i] = 0;
  }
  show(RUSAGE_SELF, "bloat-fe after malloc");
  showmaxrss();
}

Dockerfile

FROM debian:buster-slim

RUN apt-get update && apt-get install -y build-essential

COPY . /app
WORKDIR /app

RUN make clean

Makefile

.PHONY: all
all: show-maxrss bloat-fe

clean:
	rm -f show-maxrss bloat-fe *.o

HS = show-maxrss-lib.h runmaxrss.h

show-maxrss: show-maxrss.c $(HS) show-maxrss-lib.o
	$(CC) -o show-maxrss show-maxrss.c show-maxrss-lib.o
bloat-fe: bloat-fe.c $(HS) runmaxrss.o show-maxrss-lib.o
	$(CC) -o bloat-fe bloat-fe.c runmaxrss.o show-maxrss-lib.o

.c.o:
	$(CC) -c $<

runmaxrss.c

#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <sys/resource.h>

void showmaxrss() {
  pid_t pid = fork();
  printf("f=>%d\n", pid); // For some reason, this makes execl succeed. ??
  if (pid < 0) {
    printf("error running fork: errno=%d\n", errno);
    exit(1);
  }
  if (pid == 0) {
    int res = execl("./show-maxrss", "show-maxrss");
    printf("error running exec: errno=%d\n", errno);
    exit(1);
  } else {
    int stat_loc;
    struct rusage usage;
    wait4(pid, &stat_loc, 0, &usage);
    printf("child ru_maxrss: %ld\n", usage.ru_maxrss);
  }
}

runmaxrss.h

void showmaxrss();

shell

#!/bin/bash

set -ex

docker build --rm --force-rm -t ru-maxrss .
docker run -it --rm ru-maxrss

show-maxrss-lib.c

#include <errno.h>
#include <sys/resource.h>
#include <stdio.h>

void show(int who, const char *whos) {
  int res;
  struct rusage usage;
  res = getrusage(who, &usage);
  if (res != 0) {
    printf("%s: error: errno=%d\n", whos, errno);
  } else {
    printf("%s: ru_maxrss = %ld\n", whos, usage.ru_maxrss);
  }
}

show-maxrss-lib.h

void show(int who, const char *whos);

show-maxrss.c

#include <sys/resource.h>
#include "show-maxrss-lib.h"

#define _show(who) show(who, #who)

int main() {
  _show(RUSAGE_SELF);
  _show(RUSAGE_CHILDREN);
#ifdef RUSAGE_THREAD
  _show(RUSAGE_THREAD);
#endif
  return 0;
}