Approach

I l @ ve RuBoard

To assess COW, I performed measurements of three kinds of functions:

1. Copying (where COW shines, its raison d' tre )

2. Mutating operations that could trigger reallocation (represented by Append() , which gradually lengthens; this is to make sure any conclusions drawn can take into account periodic reallocation overhead due to normal string use)

3. Possibly-mutating operations that do not change length enough to trigger reallocation, or that do not actually mutate the string at all (represented by operator[]() )

It turns out that the last two both incur a constant (and similar within about 20%) cost per operation, and can roughly be considered together. Assuming for simplicity that mutating-and-extending operations, such as Append() (235ms overhead), and possibly-mutating operations like operator[]() (280ms overhead), will be about equally frequent, the COW_AtomicInt overhead for mutating and possibly-mutating operations is about 260ms per 1,000,000 operations in this implementation.

Finally, for each of 2(a) and 2(b), I first used the data shown under Raw Measurements below to hand-calculate a rough prediction of expected relative performance, then ran the test to check actual performance.

 SUMMARY FOR CASE 2(a):   PREDICTION     COW_AtomicInt Cost         Plain Cost     -------------------------------------------------     1M shallow copies          1M deep copies      and dtors            400   and dtors        1600     667K mutations        ???                     ???     667K deep copies     1060     extra overhead on      667K deep copies     ???     extra overhead on      667K mutations       175                          -----                  ------                          1635+                   1600+   TEST       (program that makes copies in a tight loop, and        modifies 33% of them with a single Append and        another 33% of them with a single op[])     Running 1000000 iterations with strings of length 50:       Plain_FastAlloc    642ms  copies: 1000000  allocs: 1000007                 Plain   1726ms  copies: 1000000  allocs: 1000007            COW_Unsafe   1629ms  copies: 1000000  allocs:  666682         COW_AtomicInt   1949ms  copies: 1000000  allocs:  666682        COW_AtomicInt2   1925ms  copies: 1000000  allocs:  666683           COW_CritSec  10660ms  copies: 1000000  allocs:  666682             COW_Mutex  33298ms  copies: 1000000  allocs:  666682 SUMMARY FOR CASE 2(b):   PREDICTION     COW_AtomicInt Cost         Plain Cost    --------------------------------------------------     1M shallow copies          1M deep copies      and dtors            400   and dtors        1600     1.5M mutations        ???                     ???     500K deep copies      800     extra overhead on      500K deep copies     ???     extra overhead on      1.5M mutations       390                          -----                  ------                          1590+                   1600+     TEST         (program that makes copies in a tight loop, and          modifies 25% of them with three Appends and          another 25% of them with three operator[]s)       Running 1000000 iterations with strings of length 50:         Plain_FastAlloc    683ms  copies: 1000000  allocs: 1000007                   Plain   1735ms  copies: 1000000  allocs: 1000007              COW_Unsafe   1407ms  copies: 1000000  allocs:  500007           COW_AtomicInt   1838ms  copies: 1000000  allocs:  500007          COW_AtomicInt2   1700ms  copies: 1000000  allocs:  500008             COW_CritSec   8507ms  copies: 1000000  allocs:  500007               COW_Mutex  31911ms  copies: 1000000  allocs:  500007 

I l @ ve RuBoard