Re: [RFC] nvfs: a filesystem for persistent memory

[Dan Williams]

On Mon, Sep 21, 2020 at 9:19 AM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
>
>
> On Tue, 15 Sep 2020, Dan Williams wrote:
>
> > > TODO:
> > >
> > > - programs run approximately 4% slower when running from Optane-based
> > > persistent memory. Therefore, programs and libraries should use page cache
> > > and not DAX mapping.
> >
> > This needs to be based on platform firmware data f(ACPI HMAT) for the
> > relative performance of a PMEM range vs DRAM. For example, this
> > tradeoff should not exist with battery backed DRAM, or virtio-pmem.
>
> Hi
>
> I have implemented this functionality - if we mmap a file with
> (vma->vm_flags & VM_DENYWRITE), then it is assumed that this is executable
> file mapping - the flag S_DAX on the inode is cleared on and the inode
> will use normal page cache.
>
> Is there some way how to test if we are using Optane-based module (where
> this optimization should be applied) or battery backed DRAM (where it
> should not)?

No, there's no direct reliable type information. Instead the firmware
on ACPI platforms provides the HMAT table which provides performance
details of system-memory ranges.