RFR: 8364248: Separate memory limit detection [v3]

[Stefan Karlsson]

On Tue, 29 Jul 2025 13:42:40 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

>> The function os::has_allocatable_memory_limit() is intended to determine whether there is a system-imposed limit on how much memory can be committed, and if so, what that limit is. On POSIX systems, limiting committable memory is typically enforced by restricting the available virtual address space, such as via RLIMIT_AS. As a result, os::has_allocatable_memory_limit() tells us both how much memory can be committed and how much virtual address space is available. On Windows however, os::has_allocatable_memory_limit() always returns true, along with the size of the available virtual address space. This is misleading because it is not possible to limit how much memory can be committed via virtual address space, and also the virtual address space cannot be limited.
>> 
>> ZGC currently uses os::has_allocatable_memory_limit() to check if the virtual address space is limited. To make it clear that the virtual address space cannot be limited on Windows, I propose that we create a new function called os::has_limited_virtual_address_space() which simply returns false on Windows, since the virtual address space cannot be limited there.
>> 
>> As a follow-up, I think it is reasonable to re-visit the implementation of os::has_allocatable_memory_limit() on Windows, since it doesn't follow any user-set limits, apart from how much virtual memory is available. Perhaps looking at limit(s) set by Job Objects could be more fruitful, and would improve the support for native Windows containers (Hyper-V).
>> 
>> Testing:
>> * Oracle's tier1-2
>> * Manual testing on Linux by limiting the virtual address space:
>> 
>> $ ulimit -v 8388608 && java -XX:+UseZGC -Xlog:gc+init -version
>
> Joel Sikström has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Rename os::has_allocatable_memory_limit() to os::allocatable_memory_limit()

src/hotspot/os/posix/os_posix.cpp line 738:

> 736:   // On POSIX systems, the amount of allocatable memory is limited by the
> 737:   // size of the virtual address space.
> 738:   *limit = address_space_limit();

In @toxaart's PR the out parameter is never set if the return value is false. I think the reason why that was done was so that code could be written like this:

size_t value = 0;
if (os::memory_function(value)) {
  // Succeeded - do stuff
} else {
  // Failed - do other stuff
}

// Use 0 if the function returned an error
use(value);


I think we should strive for unification around that in the various os:: memory functions. I also think that he used `&` in his patch and this function uses a pointer with the output parameter. Unification around would be good as well.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26530#discussion_r2240073711