Allow setting cache size based on memory percentage (#184)

The problem:

1. For the most people imlib2's default 4MiB is unreasonably low;

2. Hardcoding cache size to ~256MiB has performance benefits and doesn't
   increase RAM usage too much on relatively modern systems;

3. But we can't do that, because that would be detrimental to low spec systems
   that (apparently) not (?) building nsxiv from source, as been discussed
   #171

Solution:

Calculate cache size based on total memory.

Default is set as 3%, which means:
* ~245MiB for 8GiB
* ~30MiB for 1GiB
* and so on

CACHE_SIZE_LIMIT (256MiB by default) sets the highest possible value. And in
case we cannot determine the total memory (e.g since _SC_PHYS_PAGES isn't
POSIX) use CACHE_SIZE_FALLBACK (32MiB by default) instead.

Co-authored-by: NRK <nrk@disroot.org>

1 files changed, 19 insertions, 1 deletions

diff --git a/image.c b/image.c
index 2cec163..800321a 100644
--- a/image.c
+++ b/image.c
@@ -46,12 +46,30 @@ enum { DEF_WEBP_DELAY = 75 };
 #define ZOOM_MIN (zoom_levels[0] / 100)
 #define ZOOM_MAX (zoom_levels[ARRLEN(zoom_levels)-1] / 100)
 
+static int calc_cache_size(void)
+{
+	int cache;
+	long pages, page_size;
+
+	if (CACHE_SIZE_MEM_PERCENTAGE <= 0)
+		return 0;
+
+	pages = sysconf(_SC_PHYS_PAGES);
+	page_size = sysconf(_SC_PAGE_SIZE);
+	if (pages < 0 || page_size < 0)
+		return CACHE_SIZE_FALLBACK;
+	cache = (pages/100) * CACHE_SIZE_MEM_PERCENTAGE;
+	cache *= page_size;
+
+	return MIN(cache, CACHE_SIZE_LIMIT);
+}
+
 void img_init(img_t *img, win_t *win)
 {
 	imlib_context_set_display(win->env.dpy);
 	imlib_context_set_visual(win->env.vis);
 	imlib_context_set_colormap(win->env.cmap);
-	imlib_set_cache_size(CACHE_SIZE);
+	imlib_set_cache_size(calc_cache_size());
 
 	img->im = NULL;
 	img->win = win;