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the entire intent of using madvise/MADV_FREE on freed slots is to
improve system performance by avoiding evicting cache of useful data,
or swapping useless data to disk, by marking any whole pages in the
freed slot as discardable by the kernel. in particular, unlike
unmapping the memory or replacing it with a PROT_NONE region, use of
MADV_FREE does not make any difference to memory accounting for commit
charge purposes, and so does not increase the memory available to
other processes in a non-overcommitted environment.
however, various measurements have shown that inordinate amounts of
time are spent performing madvise syscalls in processes which
frequently allocate and free medium sized objects in the size range
roughly between PAGESIZE and MMAP_THRESHOLD, to the point that the net
effect is almost surely significant performance degredation. so, turn
it off.
the code, which has some nontrivial logic for efficiently determining
whether there is a whole-page range to apply madvise to, is left in
place so that it can easily be re-enabled if desired, or later tuned
to only apply to certain sizes or to use additional heuristics.
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as an outcome of Austin Group issue #385, future versions of the
standard will require free not to alter the value of errno. save and
restore it individually around the calls to madvise and munmap so that
the cost is not imposed on calls to free that do not result in any
syscall.
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the files added come from the mallocng development repo, commit
2ed58817cca5bc055974e5a0e43c280d106e696b. they comprise a new malloc
implementation, developed over the past 9 months, to replace the old
allocator (since dubbed "oldmalloc") with one that retains low code
size and minimal baseline memory overhead while avoiding fundamental
flaws in oldmalloc and making significant enhancements. these include
highly controlled fragmentation, fine-grained ability to return memory
to the system when freed, and strong hardening against dynamic memory
usage errors by the caller.
internally, mallocng derives most of these properties from tightly
structuring memory, creating space for allocations as uniform-sized
slots within individually mmapped (and individually freeable)
allocation groups. smaller-than-pagesize groups are created within
slots of larger ones. minimal group size is very small, and larger
sizes (in geometric progression) only come into play when usage is
high.
all data necessary for maintaining consistency of the allocator state
is tracked in out-of-band metadata, reachable via a validated path
from minimal in-band metadata. all pointers passed (to free, etc.) are
validated before any stores to memory take place. early reuse of freed
slots is avoided via approximate LRU order of freed slots. further
hardening against use-after-free and double-free, even in the case
where the freed slot has been reused, is made by cycling the offset
within the slot at which the allocation is placed; this is possible
whenever the slot size is larger than the requested allocation.
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