Question

In Java, a weak reference is garbage collected if memory out. In Linux, malloc() always returns a strong reference, ie. the pointer is never freed until the caller call free() function.

I want to allocate a buffer for caching, which could be freed automatically when the memory is running out, like following:

cache_t cache;
if (! cache_alloc(&cache))
    die("Memory out");

cache_lock(&cache); // realloc cache mem if it is collected

if (! cache->user_init) { // The "user_init" maybe reset if the cache mem is collected
  // lazy-init the cache...
  load_contents(cache->mem, ...);
  cache->user_init = 1;
}

// do with cache..
stuff_t *stuff = (stuff_t *) cache->mem;
...

cache_unlock(&cache);

It seems the buff and cache in the output of vmstat is disk IO related:

$ vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0  51604 554220  13384 314852    3   10   411   420  702 1063  8  3 75 14

Well, I want to know more about whether the cache in my example could be reflected in the “cache” column in the vmstat output.

Answer 1

There really isn’t a good way of doing it – the C memory model simply doesn’t allow for the same kind of behavior that the Java memory model allows. Java’s memory model of course builds on the C model when interfacing with the operating system, which is why the Java heap must be manually limited by the application launcher.

The “buff” and “cache” columns relate to the page/disk cache and internal buffers used by the kernel. These caches are automatically handled by the kernel – for instance, reading a file will place the contents in the “cache” usage number, in the same way that running out of memory will commit it to a swap device (“swpd”).