Date: Mon, 19 Sep 2022 09:46:59 -0400 From: Rich Felker <dalias@...c.org> To: Florian Weimer <fweimer@...hat.com> Cc: baiyang <baiyang@...il.com>, musl <musl@...ts.openwall.com> Subject: Re: The heap memory performance (malloc/free/realloc) is significantly degraded in musl 1.2 (compared to 1.1) On Mon, Sep 19, 2022 at 02:36:41PM +0200, Florian Weimer wrote: > * Szabolcs Nagy: > > > unlike musl those implementations don't return exact size nor have the > > same security and memory fragmentation guarantees, so bad comparision. > > > > tcmalloc: > > // Returns the actual number N of bytes reserved by tcmalloc for the pointer > > // p. This number may be equal to or greater than the number of bytes > > // requested when p was allocated. > > // > > // This function is just useful for statistics collection. The client must > > // *not* read or write from the extra bytes that are indicated by this call. > > > > jemalloc: > > <para>The <function>malloc_usable_size()</function> function > > returns the usable size of the allocation pointed to by > > <parameter>ptr</parameter>. The return value may be larger than the size > > that was requested during allocation. The > > <function>malloc_usable_size()</function> function is not a > > mechanism for in-place <function>realloc()</function>; rather > > it is provided solely as a tool for introspection purposes. Any > > discrepancy between the requested allocation size and the size reported > > by <function>malloc_usable_size()</function> should not be > > depended on, since such behavior is entirely implementation-dependent. > > These implementations are buggy or at least mis-documented. The > interface contract is clearly that for that particular object, the extra > bytes in the allocation are available for reading and writing. It is > not guaranteed that the allocator will always provide the same number of > extra bytes for the same requested size, but they must be there for the > allocation being examined. It's even in the name of the function! I'm not sure I understand what you're saying, but the core problem that really can't be solved is potential discrepancy between the malloc implementation's idea of usable and the compiler's. For example: char *p = malloc(1); if (malloc_usable_size(p)>1) p = 42; will cause a compiler that's actively detecting UB to abort the program when malloc_usable_size returns a value larger than 1. Rich
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