Implementing VOP_READPLUS() in FreeBSD 15?

This is a followup to a discussion with the nfs-ganesha developers.

Could FreeBSD implement a VOP_READPLUS() in FreeBSD 15, please?

Citing Lionel Cons/CERN:
> But the point is to optimise the read(). First, you have less traffic over the wire (which is a
> thing if your reads are in the gigabyte range for large VMs), and it tells the VM host that it
> can just map all those MMU pages representing the hole to the "default zero page", which
> in turn saves lots of space in the L3 and L2 caches ----> THIS DOES WONDERS to VM
> performance.
>
> Example:
> The performance benefit here comes from the fast that instead of mapping a 1TB hole
> (1099511627776 bytes) to individual 524288 2M pages (x86 2M hugepage size), and then
> potentially reading from them, you just have ONE 2M page in the cache, and all reads come
> from that.
>
> READ_PLUS is THE game changer for that kind of application, especially in our case (HPC
> simulations).

I just played with that:

1. Intel XEON with 512GB
2. loading 16 files with 64GB sparse files which are only holes
3. create kernel core dump
Result: Almost all pages in the file cache are zero bytes.

VOP_READPLUS() would optimize this case, and map all ranges belonging
to sparse file holes into the same read-only MMU page representing a
physical address range containing zero bytes. Because it's the same
physical memory it would consume very little L2/L3 cache space, and
save space in the filesystem cache too.

Aurélien
-- 
Aurélien Couderc <aurelien.couderc2002@gmail.com>
Big Data/Data mining expert, chess enthusiast