Re: [Patch] libgomp: Use libnuma for OpenMP's partition=nearest allocation trait

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

On Wed, 12 Jul 2023 at 17:35, Tobias Burnus <tobias@codesourcery.com> wrote:
>
> Now committed as r14-2462-g450b05ce54d3f0.
Hi Tobias,
The newly added tests in above commit -- alloc-11.c and alloc-12.c
seem to fail during execution
on armv8l-unknown-linux-gnueabihf:

Running libgomp:libgomp.c++/c++.exp ...
FAIL: libgomp.c++/../libgomp.c-c++-common/alloc-11.c execution test
FAIL: libgomp.c++/../libgomp.c-c++-common/alloc-12.c execution test

Running libgomp:libgomp.c/c.exp ...
FAIL: libgomp.c/../libgomp.c-c++-common/alloc-11.c execution test
FAIL: libgomp.c/../libgomp.c-c++-common/alloc-12.c execution test

Could you please investigate ?

Thanks,
Prathamesh
>
> Changes to the patch in previous email:
> * I fixed some issues found on the way,
> * The wording in the .texi has been improved/expanded, and
> * I included two testcases to exercise the two libraries (or
>    the default allocator when it is not available at runtime).
>
> Given that the default allocation already works fine (nearest)
> and the normal "malloc" is more economic in terms of memory
> handling (not multiples of page size or requesting a fixed
> pool size), I was wondering whether this patch is really needed.
>
> But at the end: default can be changed (cf. below) and given
> the user the choice makes sense. The manual states what GCC does
> which should help to make a conscious choice.
>
> * * *
>
> I did experiment with the testcase attached to previous email
> plus using dlopen to obtain the functions from libnuma if available.
>
> It was also using:
> /* { dg-do run { target { dlopen } } } */
> /* { dg-additional-options "-ldl" } */
>
> However, the Linux kernel too often placed the allocated memory
> on the "wrong" node to be usable as a testcase. I did get be
> 0 to 15 misplaced allocations, depending on the run.
>
> Hence, there is no such testcase. Using numactrl --preferred=1 I
> could force the normal allocation to (mostly) use node 1 for
> allocations such that the difference between partiton = default/environment
> vs. partition = nearest was clearly visible. Hence it does work.
>
> Otherwise, the same applies as I wrote the yesterday:
>
> On 11.07.23 12:35, Tobias Burnus wrote:
>
> > While by default 'malloc' allocates memory on the same node as the
> > calling
> > process/thread ('numactl --show' shows 'preferred node: current',
> > Linux kernel memory policy MPOL_DEFAULT), this can be changed.
> > For instance, when running the program as follows, 'malloc' now
> > prefers to allocate on the second node:
> >   numactl --preferred=1 ./myproc
> >
> > Thus, it seems to be sensible to provide a means to ensure the 'nearest'
> > allocation.  The MPOL_LOCAL policy does so, as provided by
> > libnuma's numa_alloc_local. (Which is just wrapper around the syscalls
> > mmap and mbind.) As with (lib)memkind, there is a run-time dlopen check
> > for (lib)numa - and no numa*.h is required when bulding GCC.
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