• d-bugmail puremagic.com (33/33) Jan 11 https://issues.dlang.org/show_bug.cgi?id=24329

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=24329

          Issue ID: 24329
           Summary: GC may not leave any memory for C
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Keywords: industry
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody puremagic.com
          Reporter: dlang-bugzilla thecybershadow.net

When the GC tries to allocate some memory and has none available in the pools
it already acquired from the OS, it uses a heuristic to decide whether to
trigger a collection cycle or to just request more memory from the OS.

There are some allocation patterns that may cause it to request the last bit of
memory from the OS, even though a collection could otherwise satisfy that
demand for memory. When this happens, no more memory can be allocated from
anywhere in the program, including from C code using malloc.

Though programs generally do not "run out of memory" on modern systems, due to
the prevalence of virtual memory, it can still happen if the virtual memory
size is limited by `ulimit -v`, or the program runs in a container/cgroup which
imposes a memory limit.

This becomes a problem when trying to interoperate with C libraries (as most D
programs ultimately do). For example, one symptom of this is std.net.curl
requests failing with "Out of memory on handle XXXXXXXXX".

Ideally, the GC should stop allocating (if it can avoid doing so) when the
total program memory usage approaches the total program memory limit, though
querying these two figures may not be straight-forward on all platforms.

https://github.com/dlang/dmd/blob/4fc086b0c9d08a5bcdabecdec441ec779462181e/druntime/src/core/internal/gc/os.d#L213-L215

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