memkind with devdax

[Daniel Waddington]

Hi,
I'm using memkind with fsdax but can't seem to get it to work with devdax?  Is there a trick to doing this?

Daniel

[Piotr Balcer]

Hi,

I'm afraid there's no trick, it's simply not supported. libmemkind uses temporary files on fsdax namespaces to allocate memory.

I don't see any fundamental reason why, technically speaking, we couldn't implement devdax support.

Address space fragmentation might be a concern.

Feel free to create a feature request if you think this would be useful:

https://github.com/memkind/memkind/issues

Piotr

[Daniel Waddington]

Hi Piotr,
Do you know if dax supports 1GiB pages on x86-64 or only 2MiB?

Daniel

[Ross Zwisler]

Filesystem DAX (fsdax) mode supports only 4k and 2MiB pages.  This is done opportunistically, meaning FS DAX will give you the largest page size based on many factors (the alignment of the mapping, the size of the block allocation from the filesystem, etc.).  You can improve your changes of getting 2MiB pages by following the steps here: https://nvdimm.wiki.kernel.org/2mib_fs_dax

Device DAX (devdax) supports 4k, 2MiB and 1GiB.  These allocations are guaranteed to be of a given size.

Please see http://pmem.io/ndctl/ndctl-create-namespace.html for more info about the fsdax and devdax modes and their various options.

- Ross