Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

[David Hildenbrand]

On 16.08.21 16:27, Matthew Wilcox wrote:
> On Mon, Aug 16, 2021 at 04:10:28PM +0200, David Hildenbrand wrote:
> > > > > Until recently, the CPUs only having 4 1GB TLB entries.  I'm sure we
> > > > > still have customers using that generation of CPUs.  2MB pages perform
> > > > > better than 1GB pages on the previous generation of hardware, and I
> > > > > haven't seen numbers for the next generation yet.
> > > >
> > > > I read that somewhere else before, yet we have heavy 1 GiB page users,
> > > > especially in the context of VMs and DPDK.
> > >
> > > I wonder if those users actually benchmarked.  Or whether the memory
> > > savings worked out so well for them that the loss of TLB performance
> > > didn't matter.
> >
> > These applications are extremely performance sensitive (i.e., RT workloads),
>
> "real time does not mean real fast".  it means predictable latency.

I know, but that doesn't explain why you would use 2 MB vs 1 GiB.

(most of these applications want also a low predictable latency)

> > > > I will rephrase my previous statement "hugetlbfs just doesn't raise these
> > > > problems because we are special casing it all over the place already". For
> > > > example, not allowing to swap such pages. Disallowing MADV_DONTNEED. Special
> > > > hugetlbfs locking.
> > >
> > > Sure, that's why I want to drag this feature out of "oh this is a
> > > hugetlb special case" and into "this is something Linux supports".
> >
> > I would have understood the move to optimize SHMEM internally - similar to
> > how we seem to optimize hugetlbfs SHMEM right now internally. (although
> > sharing page tables for shmem can still be quite tricky)
> >
> > I did not follow why we have to play games with MAP_PRIVATE, and having
> > private anonymous pages shared between processes that don't COW, introducing
> > new syscalls etc.
>
> It's not about SHMEM, it's about file-backed pages on regular
> filesystems.  I don't want to have XFS, ext4 and btrfs all with their
> own implementations of ARCH_WANT_HUGE_PMD_SHARE.

Let me ask this way: why do we have to play such games with MAP_PRIVATE?

--
Thanks,

David / dhildenb