32 lines
1.8 KiB
Text
32 lines
1.8 KiB
Text
this file documents dynamic memory allocation and deallocation in userspace.
|
|
the unused areas of a process's usable mapped memory are divided into "chunks"
|
|
with a start and a length, satisfying the following properties:
|
|
a) the length of a chunk is a positive power of 2
|
|
b) the start of a chunk is a multiple of its length
|
|
c) let s be a power of 2 and k be an integer. there are never two chunks with
|
|
length s and starts s * (2 * k) and s * (2 * k + 1). if ever an operation
|
|
would result in two such chunks, they are combined into one chunk with
|
|
length 2 * s and start 2 * s * k.
|
|
|
|
a "chunk info page" is divided into 512 64-bit integers describing up to 255
|
|
chunks and a pointer to another chunk info page. for each n from 0 to 254:
|
|
if the (2 * n)'th integer (where the first integer is the 0th one) is 0:
|
|
the (2 * n + 1)'th integer is unused.
|
|
if the (2 * n)'th integer is not 0:
|
|
the (2 * n)'th integer describes the length of a chunk
|
|
the (2 * n + 1)'th integer describes the start of the same chunk
|
|
the 510th integer is a pointer to the next chunk info page, and the 511th
|
|
integer is never used.
|
|
|
|
when a program calls new or malloc with needed size s, we find a free chunk of
|
|
length at least s + 8. we then remove that chunk from the list of free chunks,
|
|
and add back in whatever is left after the first s + 8 bytes, if anything.
|
|
in the first 8 bytes of the original chunk, we store the value s + 8. the
|
|
remainder of the orginal chunk is returned to the program.
|
|
|
|
during that process, if there isn't a chunk with the needed size, one or more
|
|
new pages are requested from the kernel to create such a chunk.
|
|
|
|
when a program calls delete or free with pointer ptr, we read the integer in
|
|
the 8 bytes starting at ptr - 8 into a variable s. we then add the region
|
|
starting at ptr - 8 with length s to the free memory.
|