• Simon Marlow's avatar
    Implement stack chunks and separate TSO/STACK objects · f30d5273
    Simon Marlow authored
    This patch makes two changes to the way stacks are managed:
    
    1. The stack is now stored in a separate object from the TSO.
    
    This means that it is easier to replace the stack object for a thread
    when the stack overflows or underflows; we don't have to leave behind
    the old TSO as an indirection any more.  Consequently, we can remove
    ThreadRelocated and deRefTSO(), which were a pain.
    
    This is obviously the right thing, but the last time I tried to do it
    it made performance worse.  This time I seem to have cracked it.
    
    2. Stacks are now represented as a chain of chunks, rather than
       a single monolithic object.
    
    The big advantage here is that individual chunks are marked clean or
    dirty according to whether they contain pointers to the young
    generation, and the GC can avoid traversing clean stack chunks during
    a young-generation collection.  This means that programs with deep
    stacks will see a big saving in GC overhead when using the default GC
    settings.
    
    A secondary advantage is that there is much less copying involved as
    the stack grows.  Programs that quickly grow a deep stack will see big
    improvements.
    
    In some ways the implementation is simpler, as nothing special needs
    to be done to reclaim stack as the stack shrinks (the GC just recovers
    the dead stack chunks).  On the other hand, we have to manage stack
    underflow between chunks, so there's a new stack frame
    (UNDERFLOW_FRAME), and we now have separate TSO and STACK objects.
    The total amount of code is probably about the same as before.
    
    There are new RTS flags:
    
       -ki<size> Sets the initial thread stack size (default 1k)  Egs: -ki4k -ki2m
       -kc<size> Sets the stack chunk size (default 32k)
       -kb<size> Sets the stack chunk buffer size (default 1k)
    
    -ki was previously called just -k, and the old name is still accepted
    for backwards compatibility.  These new options are documented.
    f30d5273
Compact.c 23.4 KB