[boost] Smart Pointer Timing Test Results

From: Gavin Collings (gcollings_at_[hidden])
Date: 2000-01-31 11:58:38

Here are the timing test results.

Initial refers to the default constructor call which would need to
allocate reference counts, for example.  Copy op refers to half an
assignment plus half a copy-cons.  "Fast Shared" refers to Dave's
version of shared_ptr using a special purpose allocator.

All times in ns and refer to the overhead over and above the equivalent
raw pointer operation.

GCC 2.95.2 (-O3 -DNDEBUG)

          Linked        Shared         Fast Shared   Shared In    
Cyclic
Initial  -180 +/- 150   2750 +/- 140   410 +/- 110    45 +/- 120   420
+/- 60
Copy Op   280 +/-  25    260 +/-  20   235 +/-  20   130 +/-  20   313
+/- 10

Visual C 6.0 SP 3 ("Release" mode)

          Linked        Shared         Fast Shared   Shared In    
Cyclic
Initial  -330 +/- 120   1860 +/- 140   540 +/- 114  -160 +/- 70    140
+/- 70
Copy Op   140 +/-  20     90 +/-  23    55 +/-  19    68 +/- 12    140
+/- 12

You'll see that some of the initialisation values are negative.  This
is due to the difficulty in accurately backing out the overhead -
different optimisations etc.  The same numbers have been taken away
from each type of pointer though, so the differences between these
numbers should be valid - i.e. they can be used to calculate number of
copy operations trade-offs.

So for example, in Visual C++ fast_shared_in_ptr becomes more efficient
than linked_ptr at: -

        (540 - (-330)) / (140 - 55)  = about 10 copy operations.

Method (for those interested )

The test involves iterating a loop which creates raw pointers, which it
then shares among a varying number (set size) of smart pointers.  The
same operations are then performed on raw pointers to get an idea of
unavoidable computation.  The raw pointer figure is subtracted leaving
(hopefully) just the overhead.  A range of set sizes was used and then
a curve fitted to get a linear relation with number of initialisations
and copy-ops.

The code is now in the doc vault - subdirectory smart_pointers.  All
the results are there, the program calculates most of the numbers, but
doesn't fit a line to them.  You can use the spreadsheets to do that (I
know, Microsoft Proprietary Excel, but you can ignore them too).

Gavin.