From: David Hildenbrand <david@redhat.com>
To: Mike Kravetz <mike.kravetz@oracle.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc: Michal Hocko <mhocko@suse.com>, Peter Xu <peterx@redhat.com>,
Naoya Horiguchi <naoya.horiguchi@linux.dev>,
"Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
Andrea Arcangeli <aarcange@redhat.com>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Prakash Sangappa <prakash.sangappa@oracle.com>,
James Houghton <jthoughton@google.com>,
Mina Almasry <almasrymina@google.com>,
Ray Fucillo <Ray.Fucillo@intersystems.com>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 0/5] hugetlb: Change huge pmd sharing
Date: Fri, 8 Apr 2022 11:26:03 +0200 [thread overview]
Message-ID: <dcda550d-a92a-c95e-bd08-c578924d7f8d@redhat.com> (raw)
In-Reply-To: <4ddf7d53-db45-4201-8ae0-095698ec7e1a@oracle.com>
>>
>> Let's assume a 4 TiB device and 2 MiB hugepage size. That's 2097152 huge
>> pages. Each such PMD entry consumes 8 bytes. That's 16 MiB.
>>
>> Sure, with thousands of processes sharing that memory, the size of page
>> tables required would increase with each and every process. But TBH,
>> that's in no way different to other file systems where we're even
>> dealing with PTE tables.
>
> The numbers for a real use case I am frequently quoted are something like:
> 1TB shared mapping, 10,000 processes sharing the mapping
> 4K PMD Page per 1GB of shared mapping
> 4M saving for each shared process
> 9,999 * 4M ~= 39GB savings
3.7 % of all memory. Noticeable if the feature is removed? yes. Do we
care about supporting such corner cases that result in a maintenance
burden? My take is a clear no.
>
> However, if you look at commit 39dde65c9940c which introduced huge pmd sharing
> it states that performance rather than memory savings was the primary
> objective.
>
> "For hugetlb, the saving on page table memory is not the primary
> objective (as hugetlb itself already cuts down page table overhead
> significantly), instead, the purpose of using shared page table on hugetlb is
> to allow faster TLB refill and smaller cache pollution upon TLB miss.
>
> With PT sharing, pte entries are shared among hundreds of processes, the
> cache consumption used by all the page table is smaller and in return,
> application gets much higher cache hit ratio. One other effect is that
> cache hit ratio with hardware page walker hitting on pte in cache will be
> higher and this helps to reduce tlb miss latency. These two effects
> contribute to higher application performance."
>
> That 'makes sense', but I have never tried to measure any such performance
> benefit. It is easier to calculate the memory savings.
It does makes sense; but then, again, what's specific here about hugetlb?
Most probably it was just easy to add to hugetlb in contrast to other
types of shared memory.
>
>>
>> Which results in me wondering if
>>
>> a) We should simply use gigantic pages for such extreme use case. Allows
>> for freeing up more memory via vmemmap either way.
>
> The only problem with this is that many processors in use today have
> limited TLB entries for gigantic pages.
>
>> b) We should instead look into reclaiming reconstruct-able page table.
>> It's hard to imagine that each and every process accesses each and
>> every part of the gigantic file all of the time.
>> c) We should instead establish a more generic page table sharing
>> mechanism.
>
> Yes. I think that is the direction taken by mshare() proposal. If we have
> a more generic approach we can certainly start deprecating hugetlb pmd
> sharing.
My strong opinion is to remove it ASAP and get something proper into place.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2022-04-08 9:26 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-06 20:48 [RFC PATCH 0/5] hugetlb: Change huge pmd sharing Mike Kravetz
2022-04-06 20:48 ` [RFC PATCH 1/5] hugetlbfs: revert use i_mmap_rwsem to address page fault/truncate race Mike Kravetz
2022-04-06 20:48 ` [RFC PATCH 2/5] hugetlbfs: revert use i_mmap_rwsem for more pmd sharing synchronization Mike Kravetz
2022-04-06 20:48 ` [RFC PATCH 3/5] hugetlbfs: move routine remove_huge_page to hugetlb.c Mike Kravetz
2022-04-06 20:48 ` [RFC PATCH 4/5] hugetlbfs: catch and handle truncate racing with page faults Mike Kravetz
2022-04-06 20:48 ` [RFC PATCH 5/5] hugetlb: Check for pmd unshare and fault/lookup races Mike Kravetz
2022-04-07 10:08 ` [RFC PATCH 0/5] hugetlb: Change huge pmd sharing David Hildenbrand
2022-04-07 16:17 ` Mike Kravetz
2022-04-08 9:26 ` David Hildenbrand [this message]
2022-04-19 22:50 ` Mike Kravetz
2022-04-20 7:12 ` David Hildenbrand
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