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Date: Tue, 20 Sep 2022 12:59:00 -0400
From: James Y Knight <jyknight@...gle.com>
To: musl@...ts.openwall.com
Cc: Florian Weimer <fweimer@...hat.com>, Rich Felker <dalias@...c.org>, baiyang <baiyang@...il.com>
Subject: Re: The heap memory performance (malloc/free/realloc) is
significantly degraded in musl 1.2 (compared to 1.1)
On Tue, Sep 20, 2022 at 9:58 AM Siddhesh Poyarekar <siddhesh@...hat.com>
wrote:
> Adding support for something that's already declared as bad
> programming practice seems like a step backwards. Instead, I hope we
> find a way to discourage active use of malloc_usable_size more
> strongly.
BTW, if folks aren't aware, there is already work on the C++ side to expose
an API which lets you request a heap allocation of _at least_ the given
size, which rounds the actual size up in whatever way the allocator likes,
and returns the pointer and actual size allocated. With this API, you
declare an explicit intent that all of the memory -- up to the returned
size -- is valid to use without needing to go back to the allocator to ask
for more.
The proposal is still making its way through the standardization process,
but hopefully it'll make it into the next version of C++ after C++23. (Of
course, that's not a sure thing until it happens.) Here's the doc, with
more rationale/etc:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0901r9.html
Also, as noted in the doc, jemalloc experimentally implemented this
functionality in its non-standard API, via a function it called "smallocx"
-- though jemalloc hides the API so it can't be used by default. The API is
effectively:
typedef struct { void *ptr; size_t size; } smallocx_return_t;
smallocx_return_t smallocx(size_t size, int flags);
https://github.com/jemalloc/jemalloc/blob/a0734fd6ee326cd2059edbe4bca7092988a63684/src/jemalloc.c#L3414
(That's consistent with jemalloc's other non-standard APIs, which stick
alignment/etc into a "flags" argument, but probably not suitable for a
more-standardized cross-implementation API)
tcmalloc implements similar functionality, as well, with family of
functions named "tcmalloc_size_returning_operator_new":
https://github.com/google/tcmalloc/blob/267aa2ec2817ab9d09b3fbb65ecb90193dd4348e/tcmalloc/malloc_extension.h#L549
which of course also isn't a suitable API to support cross-implementation.
If someone wants to push forward this area, IMO, it would be really great
to have an API exposing this functionality designed to be implemented in a
common way across libc malloc implementations -- and eventually added to
POSIX or C.
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