What does monit consider to be memory usage

(1) I see that each of the running processes occupies a very small percentage of memory (%MEM no more than 0.2%, and most just 0.0%), but how the total memory is almost used as in the fourth line of output ("Mem: 130766620k total, 130161072k used, 605548k free, 919300k buffers")? The sum of used percentage of memory over all processes seems unlikely to achieve almost 100%, doesn't it?

To see how much memory you are currently using, run free -m. It will provide output like:

             total       used       free     shared    buffers     cached
Mem:          2012       1923         88          0         91        515
-/+ buffers/cache:       1316        695
Swap:         3153        256       2896

The top row 'used' (1923) value will almost always nearly match the top row mem value (2012). Since Linux likes to use any spare memory to cache disk blocks (515).

The key used figure to look at is the buffers/cache row used value (1316). This is how much space your applications are currently using. For best performance, this number should be less than your total (2012) memory. To prevent out of memory errors, it needs to be less than the total memory (2012) and swap space (3153).

If you wish to quickly see how much memory is free look at the buffers/cache row free value (695). This is the total memory (2012)- the actual used (1316). (2012 - 1316 = 696, not 695, this will just be a rounding issue)

(2) how to understand the load average on the first line ("load average: 14.04, 14.02, 14.00")?

This article on load average uses a nice traffic analogy and is the best one I've found so far: Understanding Linux CPU Load - when should you be worried?. In your case, as people pointed out:

On multi-processor system, the load is relative to the number of processor cores available. The "100% utilization" mark is 1.00 on a single-core system, 2.00, on a dual-core, 4.00 on a quad-core, etc.

So, with a load average of 14.00 and 24 cores, your server is far from being overloaded.

The problem is that a lot of the kernel data structures such as the page descriptors (one struct for every 4KB page in the system) need to be in low memory. So as the total memory in the machine goes up, more and more low memory is also needed, and eventually low memory becomes a very scarce resource.

IIRC the usual rule of thumb is that 16 GB total is about the upper sane limit for a 32-bit kernel. There's not very much you can do about it.

You can try to boot with less memory (mem= command line parameter to the kernel). But the real solution is to switch to a 64-bit kernel.