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- DEFAULT_TRIM_THRESHOLD default: 2MB
- Also settable using mallopt(M_TRIM_THRESHOLD, x)
- The maximum amount of unused top-most memory to keep before
- releasing via malloc_trim in free(). Automatic trimming is mainly
- useful in long-lived programs using contiguous MORECORE. Because
- trimming via sbrk can be slow on some systems, and can sometimes be
- wasteful (in cases where programs immediately afterward allocate
- more large chunks) the value should be high enough so that your
- overall system performance would improve by releasing this much
- memory. As a rough guide, you might set to a value close to the
- average size of a process (program) running on your system.
- Releasing this much memory would allow such a process to run in
- memory. Generally, it is worth tuning trim thresholds when a
- program undergoes phases where several large chunks are allocated
- and released in ways that can reuse each other's storage, perhaps
- mixed with phases where there are no such chunks at all. The trim
- value must be greater than page size to have any useful effect. To
- disable trimming completely, you can set to MAX_SIZE_T. Note that the trick
- some people use of mallocing a huge space and then freeing it at
- program startup, in an attempt to reserve system memory, doesn't
- have the intended effect under automatic trimming, since that memory
- will immediately be returned to the system.
- DEFAULT_MMAP_THRESHOLD default: 256K
- Also settable using mallopt(M_MMAP_THRESHOLD, x)
- The request size threshold for using MMAP to directly service a
- request. Requests of at least this size that cannot be allocated
- using already-existing space will be serviced via mmap. (If enough
- normal freed space already exists it is used instead.) Using mmap
- segregates relatively large chunks of memory so that they can be
- individually obtained and released from the host system. A request
- serviced through mmap is never reused by any other request (at least
- not directly; the system may just so happen to remap successive
- requests to the same locations). Segregating space in this way has
- the benefits that: Mmapped space can always be individually released
- back to the system, which helps keep the system level memory demands
- of a long-lived program low. Also, mapped memory doesn't become
- `locked' between other chunks, as can happen with normally allocated
- chunks, which means that even trimming via malloc_trim would not
- release them. However, it has the disadvantage that the space
- cannot be reclaimed, consolidated, and then used to service later
- requests, as happens with normal chunks. The advantages of mmap
- nearly always outweigh disadvantages for "large" chunks, but the
- value of "large" may vary across systems. The default is an
- empirically derived value that works well in most systems. You can
- disable mmap by setting to MAX_SIZE_T.
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