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(p200300cbc704fd000612f12ba4a226b0.dip0.t-ipconnect.de. [2003:cb:c704:fd00:612:f12b:a4a2:26b0]) by smtp.gmail.com with ESMTPSA id h9-20020a05600c350900b0038cbcbcf994sm10549617wmq.36.2022.04.08.02.26.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 08 Apr 2022 02:26:04 -0700 (PDT) Message-ID: Date: Fri, 8 Apr 2022 11:26:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [RFC PATCH 0/5] hugetlb: Change huge pmd sharing Content-Language: en-US To: Mike Kravetz , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Michal Hocko , Peter Xu , Naoya Horiguchi , "Aneesh Kumar K . V" , Andrea Arcangeli , "Kirill A . Shutemov" , Davidlohr Bueso , Prakash Sangappa , James Houghton , Mina Almasry , Ray Fucillo , Andrew Morton References: <20220406204823.46548-1-mike.kravetz@oracle.com> <045a59a1-0929-a969-b184-1311f81504b8@redhat.com> <4ddf7d53-db45-4201-8ae0-095698ec7e1a@oracle.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <4ddf7d53-db45-4201-8ae0-095698ec7e1a@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >> >> 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