[ltp] X200s, 4 RAM 64 bit or not

[D. Hugh Redelmeier]

| From: Uwe Brauer <oub.oub.oub@gmail.com>

| I upgraded to 4 RAM and I am running Kubuntu 10.04, trinity with PAE
| enabled. I am thinking to switch to 64 bit, but since still the majority
| of applications is 32 bit I am not sure that it is worth the effort. 

10.04 is really old.  I don't know what support remains (surely you
do).  If you like being so far behind the bleeding edge, I wouldn't
recommend make any changes.

If you are intending to move to a new installation, I would recommend
64-bit.

In broad strokes: you won't notice much change.  Although your
computing will probably speed up, it won't likely be enough to notice
without timing.

If your / partition was 99% full, it might become 101% full :-)
But moving from 10.04 to 14.04 would increase the space requirement
much more than changing the architecture (guess).

There are a few individual programs that need more than 1G of virtual
memory.  If you use any of them, you should go 64-bit.  (Firefox will
eat insane amounts of memory but that doesn't mean that you should let
it.)

[boring details follow; you can skip these]

The downsides:

- a slight increase in the size of program code

- pointers objects take twice as much memory.  The amount of data that
  is made up of pointers varies by program, but usually isn't that
  high.

- C "long int" objects take twice as much memory.  This usually isn't
  a large consumer of memory.

- a decrease in the number of pointer and long objects that fit in the
  memory caches.  So some workloads become slower (rare).

The upsides are:

- increased CPU speed due to amd64 instructions (as debian and
  Ubuntu call them)

  + more registers so program spends less time spilling and restoring
    registers

  + floating point is quite a bit faster

  + PAE eats a significant amount of CPU so ditching it is good

- any program that uses a lot of memory (more than 1G of virtual
  memory, as I understand it) will run better

- a program can use more than 3G (I think that's the number) of
  virtual memory

- some programs exploit the increased range of long ints (or the
  increased speed of code using long long ints).

All open-source Linux programs seem to be available in AMD64.  I don't
know about other stuff.