Add another old article about special memhog for testing cgroups

content/post/slow-memhog.md

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+---
+title: Slow memhog for testing cgroups
+date: !!timestamp '2016-10-04 00:00:00'
+tags:
+    - teaching
+---
+
+Testing the cgroup memory is not something as easy as we can think. It can't be
+only question of `malloc(100000)` in a loop, as the Linux kernel overcommit
+memory allocation: so even if we get effectively a 100000 bytes long memory
+space, this doesn't decrease the physical available memory. To do so, this
+space need to be changed pages by pages, that can be tedious to do. And quite
+uncertain, because the kernel can take advantage of the swap partition...
+
+To facilitate memory testing, there is `memhog`, part of `numactl` tools. It
+aims to allocate large memory block for testing, directly in the physical
+memory.
+
+<!-- more -->
+
+If we ask memhog to allocate 100M, 1G, 5G, 10G, ... it'll actually ask the
+kernel for such amont of memory, in no time. If there is enough memory
+available, we got our 100M, 1G, 5G, ... and the available memory is really
+decreased by such volume.
+
+In my demos, I need a slightly different behaviour, to demonstrate what happens
+as a function of time.
+
+I slightly modify `memhog` to introduce a timer between slice allocation.
+
+```c
+/* Derivative work of memhog.c from the numactl repository,
+   hosted at https://github.com/numactl/numactl.git
+
+   Copyright (C) 2016 Pierre-Olivier Mercier, EPITA.
+   Copyright (C) 2003,2004 Andi Kleen, SuSE Labs.
+
+   numactl is free software; you can redistribute it and/or
+   modify it under the terms of the GNU General Public
+   License as published by the Free Software Foundation; version
+   2.
+   numactl is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   General Public License for more details.
+   You should find a copy of v2 of the GNU General Public License somewhere
+   on your Linux system; if not, write to the Free Software Foundation,
+   Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
+
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/mman.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+
+char *
+alloc_workbuf(size_t size)
+{
+	char *ptr;
+
+	/* allocate some memory */
+	ptr = malloc(size);
+
+	/* return NULL on failure */
+	if (ptr == NULL)
+	{
+		fprintf(stderr, "Unable to malloc %zuk: %s\n", size / 1024, strerror(errno));
+		return NULL;
+	}
+
+	/* lock this buffer into RAM */
+	if (mlock(ptr, size)) {
+		free(ptr);
+		if (errno == ENOMEM)
+			fprintf(stderr, "Unable to lock memory: not enough permissions\n");
+		else
+			perror("Unable to lock memory");
+		return NULL;
+	}
+	return ptr;
+}
+
+int
+main(int argc, char *argv[])
+{
+	size_t size;
+	size_t steps = 32;
+
+	/* parse arguments */
+	if (argc < 2 || argc > 3) {
+		fprintf(stderr, "Usage: memhog <size in MB> [nb steps]\n");
+		return EXIT_FAILURE;
+	}
+
+	size = atoi(argv[1]);
+	if (argc > 2)
+	  steps = atoi(argv[2]);
+
+	size = size / steps;
+
+	/* Do stuff */
+	for (size_t i = 0; i < steps; i++)
+	{
+		if (!alloc_workbuf(size * 1000000UL))
+			break;
+
+		printf(".");
+		fflush(stdout);
+
+		usleep(80000);
+	}
+
+	printf("\n");
+	return EXIT_SUCCESS;
+}
+```
+
+You can compile those C lines with just `make`, stating your file is named `memhog.c`:
+
+```shell
+make memhog
+```