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Remove references to sbrk to grow/shrink arenas
The manual incorrectly references sbrk as the method used to grow and shrink heaps and the fact that M_TRIM_THRESHOLD and M_TOP_PAD control that behavior. In reality, a heap may be grown or shrunk through multiple methods depending on whether it is the main arena (in which case sbrk is correct) or not (in which case, there are a number of strategies including allocating an additional heap to grow an arena and/or 'mprotect' a region to make it available for allocation). Remove references to sbrk so that it covers the behavior more accurately. * manual/memory.texi (M_TOP_PAD): Remove reference to sbrk. (M_TRIM_THRESHOLD): Likewise.
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@ -1,5 +1,8 @@
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2016-10-26 Siddhesh Poyarekar <siddhesh@sourceware.org>
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* manual/memory.texi (M_TOP_PAD): Remove reference to sbrk.
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(M_TRIM_THRESHOLD): Likewise.
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* manual/memory.texi (M_ARENA_TEST): Add documentation.
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(M_ARENA_MAX): Likewise.
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* malloc/malloc.c: Remove obsolete comment.
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@ -1132,11 +1132,10 @@ This parameter can also be set for the process at startup by setting the
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environment variable @env{MALLOC_MMAP_PERTURB_} to the desired value.
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@item M_TOP_PAD
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This parameter determines the amount of extra memory to obtain from the
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system when a call to @code{sbrk} is required. It also specifies the
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number of bytes to retain when shrinking the heap by calling @code{sbrk}
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with a negative argument. This provides the necessary hysteresis in
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heap size such that excessive amounts of system calls can be avoided.
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This parameter determines the amount of extra memory to obtain from the system
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when an arena needs to be extended. It also specifies the number of bytes to
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retain when shrinking an arena. This provides the necessary hysteresis in heap
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size such that excessive amounts of system calls can be avoided.
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The default value of this parameter is @code{0}.
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@ -1145,8 +1144,7 @@ environment variable @env{MALLOC_TOP_PAD_} to the desired value.
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@item M_TRIM_THRESHOLD
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This is the minimum size (in bytes) of the top-most, releasable chunk
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that will cause @code{sbrk} to be called with a negative argument in
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order to return memory to the system.
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that will trigger a system call in order to return memory to the system.
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If this parameter is not set, the default value is set as 128 KiB and the
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threshold is adjusted dynamically to suit the allocation patterns of the
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