Why lion is slow
I think it may be related to llvm:
https://forums.macrumors.com/posts/15293794/
On Snow Leopard, the default compiler is GCC 4.2 (WITH NO LLVM)
On Lion, the default compiler is GCC-LLVM, then later Clang-LLVM (because GCC-LLVM was actually
half broken).
So the major changes from Snow Leopard to Lion is the mandatory use of LLVM.
In the case of LLVM backend producing code running in virtual machine, it has support for
grabbing chunks of memory to do memory allocation (they need to in order to do automatic
reference counting and garbage collection). Kernel bloat? Could it be that LLVM, in its support of interpreted language features, carried these baggage, which resulted in bloated and slow code even
if you are compiling static CLang or GCC code? Remember, LLVM intermediate IR
(byte code in java) is VERY FAR REMOVED from standard GCC intermediate code. It is more abstract,
to the point where you can actually run the llvm IR inside a virtual machine (no different than C# or Java).
So the process from llvm IR to regular .o or regular binary executable is not as cleancut as GCC.
In addition, LLVM takes about 5 times more main memory than GCC:
http://clang-developers.42468.n3.nab...-td683717.html
Remember in OpenGL, there were some code left in intermediate state? When that kicks in the llvm
compiler starts up. We don't know what other parts of the OSX were left in this state that REQUIRES
compilation at runtime, and JIT compilation (like Java JIT). Perhaps more and more pieces in
Snow Leopard and cumulating in full blown llvm requirement in Lion. Only in Lion was full
LLVM required everywhere. This could lead to kernel
bloat because OpenGL (driver) is near the Kernel level when running when these non-compiled
code needs to be JIT compiled. It could be other low level pieces. Remember this is 5 TIMES the required memory. So something
that normally requires 1GB would now require 5GB during runtime. A Mac Mini only has 4GB and
so are lots of earlier Mac machines. A lot of disk thrashing will occur as more things are moved
back and forth to the harddrive to accomodate that startup of the llvm virtual machine backend just
to compile, and if virtual machine is used, the amount of memory never dissipates.
