diff options
Diffstat (limited to 'malloc')
| -rw-r--r-- | malloc/malloc.c | 82 |
1 files changed, 81 insertions, 1 deletions
diff --git a/malloc/malloc.c b/malloc/malloc.c index da230d3493..890d3669e2 100644 --- a/malloc/malloc.c +++ b/malloc/malloc.c @@ -1415,6 +1415,19 @@ int __posix_memalign(void **, size_t, size_t); #endif /* + MMAP_THRESHOLD_MAX and _MIN are the bounds on the dynamically + adjusted MMAP_THRESHOLD. +*/ + +#ifndef DEFAULT_MMAP_THRESHOLD_MIN +#define DEFAULT_MMAP_THRESHOLD_MIN (128 * 1024) +#endif + +#ifndef DEFAULT_MMAP_THRESHOLD_MAX +#define DEFAULT_MMAP_THRESHOLD_MAX (8 * 1024 * 1024 * sizeof(long)) +#endif + +/* M_MMAP_THRESHOLD is the request size threshold for using mmap() to service a request. Requests of at least this size that cannot be allocated using already-existing space will be serviced via mmap. @@ -1453,12 +1466,63 @@ int __posix_memalign(void **, size_t, size_t); "large" chunks, but the value of "large" varies across systems. The default is an empirically derived value that works well in most systems. + + + Update in 2006: + The above was written in 2001. Since then the world has changed a lot. + Memory got bigger. Applications got bigger. The virtual address space + layout in 32 bit linux changed. + + In the new situation, brk() and mmap space is shared and there are no + artificial limits on brk size imposed by the kernel. What is more, + applications have started using transient allocations larger than the + 128Kb as was imagined in 2001. + + The price for mmap is also high now; each time glibc mmaps from the + kernel, the kernel is forced to zero out the memory it gives to the + application. Zeroing memory is expensive and eats a lot of cache and + memory bandwidth. This has nothing to do with the efficiency of the + virtual memory system, by doing mmap the kernel just has no choice but + to zero. + + In 2001, the kernel had a maximum size for brk() which was about 800 + megabytes on 32 bit x86, at that point brk() would hit the first + mmaped shared libaries and couldn't expand anymore. With current 2.6 + kernels, the VA space layout is different and brk() and mmap + both can span the entire heap at will. + + Rather than using a static threshold for the brk/mmap tradeoff, + we are now using a simple dynamic one. The goal is still to avoid + fragmentation. The old goals we kept are + 1) try to get the long lived large allocations to use mmap() + 2) really large allocations should always use mmap() + and we're adding now: + 3) transient allocations should use brk() to avoid forcing the kernel + having to zero memory over and over again + + The implementation works with a sliding threshold, which is by default + limited to go between 128Kb and 32Mb (64Mb for 64 bitmachines) and starts + out at 128Kb as per the 2001 default. + + This allows us to satisfy requirement 1) under the assumption that long + lived allocations are made early in the process' lifespan, before it has + started doing dynamic allocations of the same size (which will + increase the threshold). + + The upperbound on the threshold satisfies requirement 2) + + The threshold goes up in value when the application frees memory that was + allocated with the mmap allocator. The idea is that once the application + starts freeing memory of a certain size, it's highly probable that this is + a size the application uses for transient allocations. This estimator + is there to satisfy the new third requirement. + */ #define M_MMAP_THRESHOLD -3 #ifndef DEFAULT_MMAP_THRESHOLD -#define DEFAULT_MMAP_THRESHOLD (128 * 1024) +#define DEFAULT_MMAP_THRESHOLD DEFAULT_MMAP_THRESHOLD_MIN #endif /* @@ -2251,6 +2315,10 @@ struct malloc_par { int n_mmaps; int n_mmaps_max; int max_n_mmaps; + /* the mmap_threshold is dynamic, until the user sets + it manually, at which point we need to disable any + dynamic behavior. */ + int no_dyn_threshold; /* Cache malloc_getpagesize */ unsigned int pagesize; @@ -3428,6 +3496,14 @@ public_fREe(Void_t* mem) #if HAVE_MMAP if (chunk_is_mmapped(p)) /* release mmapped memory. */ { + /* see if the dynamic brk/mmap threshold needs adjusting */ + if (!mp_.no_dyn_threshold + && p->size > mp_.mmap_threshold + && p->size <= DEFAULT_MMAP_THRESHOLD_MAX) + { + mp_.mmap_threshold = chunksize (p); + mp_.trim_threshold = 2 * mp_.mmap_threshold; + } munmap_chunk(p); return; } @@ -5418,10 +5494,12 @@ int mALLOPt(param_number, value) int param_number; int value; case M_TRIM_THRESHOLD: mp_.trim_threshold = value; + mp_.no_dyn_threshold = 1; break; case M_TOP_PAD: mp_.top_pad = value; + mp_.no_dyn_threshold = 1; break; case M_MMAP_THRESHOLD: @@ -5432,6 +5510,7 @@ int mALLOPt(param_number, value) int param_number; int value; else #endif mp_.mmap_threshold = value; + mp_.no_dyn_threshold = 1; break; case M_MMAP_MAX: @@ -5441,6 +5520,7 @@ int mALLOPt(param_number, value) int param_number; int value; else #endif mp_.n_mmaps_max = value; + mp_.no_dyn_threshold = 1; break; case M_CHECK_ACTION: |