| Message ID | 20221128172646.244742-1-siddhesh@sourceware.org |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] realloc: Return unchanged if request is within usable size | expand |
Siddhesh Poyarekar via Libc-alpha <libc-alpha@sourceware.org> writes: > Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org> LGTM Reviewed-by: DJ Delorie <dj@redhat.com> > diff --git a/malloc/malloc.c b/malloc/malloc.c > index 2a61c8b5ee..ef8c794fb7 100644 > --- a/malloc/malloc.c > +++ b/malloc/malloc.c > @@ -1100,6 +1100,8 @@ static void munmap_chunk(mchunkptr p); > static mchunkptr mremap_chunk(mchunkptr p, size_t new_size); > #endif > > +static size_t musable (void *mem); Matches function which occurs later on > + /* Return the chunk as is whenever possible, i.e. there's enough usable space > + but not so much that we end up fragmenting the block. We use the trim > + threshold as the heuristic to decide the latter. */ > + size_t usable = musable (oldmem); > + if (bytes <= usable > + && (unsigned long) (usable - bytes) <= mp_.trim_threshold) > + return oldmem; Ok. > diff --git a/malloc/tst-realloc.c b/malloc/tst-realloc.c > +#include <stdint.h> Ok. > > + /* Smoke test to make sure that allocations do not move if they have enough > + space to expand in the chunk. */ > + for (size_t sz = 3; sz < 256 * 1024; sz += 2048) > + { > + p = realloc (NULL, sz); size 3, 2051, 4099... always 3 bytes more than a 2048-boundary > + if (p == NULL) > + FAIL_EXIT1 ("realloc (NULL, %zu) returned NULL.", sz); > + size_t newsz = malloc_usable_size (p); Ok. > + printf ("size: %zu, usable size: %zu, extra: %zu\n", > + sz, newsz, newsz - sz); > + uintptr_t oldp = (uintptr_t) p; > + void *new_p = realloc (p, newsz); Should always work; either we're within a few words, or within a page (mmap). Ok. > + if ((uintptr_t) new_p != oldp) > + FAIL_EXIT1 ("Expanding (%zu bytes) to usable size (%zu) moved block", > + sz, newsz); > + free (new_p); > + > + /* We encountered a large enough extra size at least once. */ > + if (newsz - sz > 1024) > + break; Ok. > + > return 0; > }
diff --git a/malloc/malloc.c b/malloc/malloc.c index 2a61c8b5ee..ef8c794fb7 100644 --- a/malloc/malloc.c +++ b/malloc/malloc.c @@ -1100,6 +1100,8 @@ static void munmap_chunk(mchunkptr p); static mchunkptr mremap_chunk(mchunkptr p, size_t new_size); #endif +static size_t musable (void *mem); + /* ------------------ MMAP support ------------------ */ @@ -3396,6 +3398,14 @@ __libc_realloc (void *oldmem, size_t bytes) if (__glibc_unlikely (mtag_enabled)) *(volatile char*) oldmem; + /* Return the chunk as is whenever possible, i.e. there's enough usable space + but not so much that we end up fragmenting the block. We use the trim + threshold as the heuristic to decide the latter. */ + size_t usable = musable (oldmem); + if (bytes <= usable + && (unsigned long) (usable - bytes) <= mp_.trim_threshold) + return oldmem; + /* chunk corresponding to oldmem */ const mchunkptr oldp = mem2chunk (oldmem); /* its size */ diff --git a/malloc/tst-realloc.c b/malloc/tst-realloc.c index 5eb62a770f..3b78a2420a 100644 --- a/malloc/tst-realloc.c +++ b/malloc/tst-realloc.c @@ -17,6 +17,7 @@ #include <errno.h> #include <malloc.h> +#include <stdint.h> #include <stdio.h> #include <string.h> #include <libc-diag.h> @@ -142,6 +143,28 @@ do_test (void) free (p); + /* Smoke test to make sure that allocations do not move if they have enough + space to expand in the chunk. */ + for (size_t sz = 3; sz < 256 * 1024; sz += 2048) + { + p = realloc (NULL, sz); + if (p == NULL) + FAIL_EXIT1 ("realloc (NULL, %zu) returned NULL.", sz); + size_t newsz = malloc_usable_size (p); + printf ("size: %zu, usable size: %zu, extra: %zu\n", + sz, newsz, newsz - sz); + uintptr_t oldp = (uintptr_t) p; + void *new_p = realloc (p, newsz); + if ((uintptr_t) new_p != oldp) + FAIL_EXIT1 ("Expanding (%zu bytes) to usable size (%zu) moved block", + sz, newsz); + free (new_p); + + /* We encountered a large enough extra size at least once. */ + if (newsz - sz > 1024) + break; + } + return 0; }
If there is enough space in the chunk to satisfy the new size, return the old pointer as is, thus avoiding any locks or reallocations. The only real place this has a benefit is in large chunks that tend to get satisfied with mmap, since there is a large enough spare size (up to a page) for it to matter. For allocations on heap, the extra size is typically barely a few bytes (up to 15) and it's unlikely that it would make much difference in performance. Also added a smoke test to ensure that the old pointer is returned unchanged if the new size to realloc is within usable size of the old pointer. Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org> --- Changes from v1: - Fixed up test as per review comments - Do the realloc bypass only when the new size is within trim_threshold bytes of the usable size so that shrinking does not hold up memory. malloc/malloc.c | 10 ++++++++++ malloc/tst-realloc.c | 23 +++++++++++++++++++++++ 2 files changed, 33 insertions(+)