linux-kernel - Re: [RFC PATCH 0/5] madvise MADV

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Message-ID: <3ce1f52f-d84d-49ba-c027-058266e16d81@redhat.com>
Date:   Mon, 16 Aug 2021 16:33:09 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>,
        "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>, Steven Sistare <steven.sistare@...cle.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

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