boost::pool question about allocation size

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stsoe
2007-04-05
2013-05-14
  • stsoe
    stsoe
    2007-04-05

    I have a question regarding the boost::pool implementation. 

    Why does boost::pool::alloc_size() have to be lcm(sizeof(void*), sizeof(size_type), requested_size)? 

    I have read the "Guaranteeing Alignment" document (http://www.boost.org/libs/pool/doc/implementation/alignment.html), and looking at the code I can see that a lot of bytes can be wasted in the chunks returned to user when requested_size is not a multiple of min_alloc_size (lcm(sizeof(void*), sizeof(size_type))).

    I would think the requested_size alignment could be the first multiple of min_alloc_size that is larger than requested_size.  For example, the convoluted example has requested_size==7, sizeof(void*)==3, and sizeof(size_type)==5.   Currently alloc_size() returns lcm(3,5,7)==105, but since min_alloc_size is lcm(3,5)==15, I would think requested_size alignment could be 15 bytes, rather than the 105 bytes.

    The Theorem in the document is still satisfied.  "For each integer i, such that pe+i is well defined, that address (pe+i) is properly aligned for each type Tn."

    Another way to look at this.   If requested_size was changed to 15, then the current implementation of alloc_size() would return 15 bytes (lcm(3,5,15)==15), so why the huge penalty when requested size is 7?

    I am probably missing something?

    Thank you.

    Soren