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
