[Octave-bug-tracker] [bug #56752] Performance slowdown from version 3.2.4 through to current dev branch

[Rik]

Follow-up Comment #15, bug #56752 (project octave):

Here is another possibility.  I find that octave_value_list is often taking
~1% of an particular leaf function.  If I check the annotated code I see that
atomic locking instructions take a very long time.


octave_value_list::octave_value_list 
/home/rik/wip/Projects_Mine/octave-dev/libgui/.libs/liboctgui.so.5.0.0
Samples│    _ZN17octave_value_listC2Ev():
       │    OCTINTERP_API
       │    octave_value_list
       │    {
       │    public:
       │
       │      octave_value_list (void)
       │      push   %rbp
    11 │      mov    %rsp,%rbp
     7 │      push   %r12
     1 │      push   %rbx
       │    _ZN17octave_value_listC1Ev():
     1 │      add    $0x10,%rax
       │    _ZN17octave_value_listC2Ev():
    15 │      mov    %rdi,%rbx
       │    _ZN17octave_value_listC1Ev():
     6 │      mov    %rax,(%rdi)
       │
       │    public:
       │
       │      static octave_idx_type dim_max (void);
       │
       │      explicit dim_vector (void) : rep (nil_rep ())
     1 │    → callq  dim_vector::nil_rep()@plt
     3 │      mov    %rax,0x8(%rbx)
       │      { OCTAVE_ATOMIC_INCREMENT (&(count ())); }
   327 │      lock   addq   $0x1,-0x10(%rax)
       │        : dimensions (), rep (nil_rep ()), slice_data (rep->data),
     1 │    → callq  Array<octave_value
       │          slice_len (rep->len)
    10 │      mov    (%rax),%rdx
     7 │      mov    %rax,0x10(%rbx)
       │      mov    %rdx,0x18(%rbx)
       │      mov    0x8(%rax),%rdx
    14 │      mov    %rdx,0x20(%rbx)
       │          return OCTAVE_ATOMIC_INCREMENT (&m_count);
       │        }
       │
       │        count_type operator++ (int)
       │        {
       │          return OCTAVE_ATOMIC_POST_INCREMENT (&m_count);
   297 │      lock   addl   $0x1,0x10(%rax)
     1 │      mov    vtable for Array<std::__cxx11::basic_string<char,
std::char_traits<char,%rax
       │      add    $0x10,%rax
    16 │      mov    %rax,0x28(%rbx)
       │      explicit dim_vector (void) : rep (nil_rep ())
       │    → callq  dim_vector::nil_rep()@plt
       │      mov    %rax,0x30(%rbx)
       │      { OCTAVE_ATOMIC_INCREMENT (&(count ())); }
   294 │      lock   addq   $0x1,-0x10(%rax)
       │        : dimensions (), rep (nil_rep ()), slice_data (rep->data),
     1 │    → callq  Array<std::__cxx11::basic_string<char,
std::char_traits<char
       │          slice_len (rep->len)



I can change the atomic instructions to ordinary ones by configuring with
--disable-atomic-refcount.  The benchmark runtime drops from 14.1 seconds to
11.6 seconds (2.5 seconds) which seems important.

The requirement for atomic refcounting was introduced by communication with
the GUI.  This brings up a hard question, is there a better way to implement
cross-thread communication?



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