exact total memory usage in linux that equals to system monitor

Question

By getting Memtotal and Memfree values from "/proc/meminfo" and subtracting them we must get the Used Memory in Linux. Most of threads and web pages that I have visited have guided through this approach to compute the Total Memory Usage in Linux. But by implementing this method, I get different results with GNOME System Monitor! My result is greater than that (approximately double). So what is the method that GNOME System Monitor uses?

Answer 1

GNOME system monitor uses libgtop to retrieve memory information for various platforms. For Linux it uses sysdeps/linux/mem.c ² where the routine is as follows:

Strings like "MemTotal" are headings in /proc/meminfo .

…    
    buf->total  = get_scaled(buffer, "MemTotal:");
    buf->free   = get_scaled(buffer, "MemFree:");
    buf->used   = buf->total - buf->free;
    buf->shared = 0;
    buf->buffer = get_scaled(buffer, "Buffers:");
    buf->cached = get_scaled(buffer, "Cached:");

    buf->user = buf->total - buf->free - buf->cached - buf->buffer;

The memory reported in the application is buf->user . More precisely in src/load-graph.cpp ¹ by:

mempercent  = (float)mem.user  / (float)mem.total;
set_memory_label_and_picker(GTK_LABEL(graph->labels.memory),
                            GSM_COLOR_BUTTON(graph->mem_color_picker),
                            mem.user, mem.total, mempercent);