Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

David Hildenbrand
2021-08-16T14:33:18Z

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