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[2003:cb:c736:ca00:b4c3:24bd:c2f5:863c]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c2a1c3994sm10960317f8f.81.2025.01.27.06.09.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Jan 2025 06:09:24 -0800 (PST) Message-ID: Date: Mon, 27 Jan 2025 15:09:23 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Direct I/O performance problems with 1GB pages To: Matthew Wilcox , linux-mm@kvack.org Cc: linux-block@vger.kernel.org, Muchun Song , Jane Chu References: From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 94REyzPgiAAbdIoWPUyZqAEBFp4ED7zQsObKQ_AEMhE_1737986966 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 939A8180008 X-Stat-Signature: zhx3cgiyie9dp6smei1j9wgemgwesozs X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1737986969-270704 X-HE-Meta: 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 WU36yA1x T2k7hQCV8I0ojgDjS+cFc6Mk7MrHVzvrlgekk3Gp5pydHAUNG/cAM7NpXorVOQk7LAdeaHjeYLbXkNf1ANEVXiqVE+41AwwiA4dJZL22CVfAA8y7nSuwJWBK0hGzzoC2QLJz9xD7DTdwrKfGi0DVMMybEeBCYYkIEL6gVPawuluux8PkTaemurRNXDqwKOT0RusAjufvuOego/+bGjWbTscQGdZRQvgYJySSrb5wExdqI4NOfaJ4Uxe4HNtxfjTIjzwDsfg86OMvkKGVYdpL9I/IdhwJ1ztjQmkYXtBn+O+Jzw9dxMZXcBDGy7/WwxIE5sGgCW/CGCH/6VgjTtD2vP49hsPTAlTpiqB3fE+f0gLdcEGRfGA7yaf7z3xxmb7MvdDbwo94l/Fttl4U= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 26.01.25 01:46, Matthew Wilcox wrote: > Postgres are experimenting with doing direct I/O to 1GB hugetlb pages. > Andres has gathered some performance data showing significantly worse > performance with 1GB pages compared to 2MB pages. I sent a patch > recently which improves matters [1], but problems remain. > > The primary problem we've identified is contention of folio->_refcount > with a strong secondary contention on folio->_pincount. This is coming > from the call chain: > > iov_iter_extract_pages -> > gup_fast_fallback -> > try_grab_folio_fast > > Obviously we can fix this by sharding the counts. We could do that by > address, since there's no observed performance problem with 2MB pages. > But I think we'd do better to shard by CPU. We have percpu-refcount.h > already, and I think it'll work. > > The key to percpu refcounts is knowing at what point you need to start > caring about whether the refcount has hit zero (we don't care if the > refcount oscillates between 1 and 2, but we very much care about when > we hit 0). > > I think the point at which we call percpu_ref_kill() is when we remove a > folio from the page cache. Before that point, the refcount is guaranteed > to always be positive. After that point, once the refcount hits zero, > we must free the folio. > > It's pretty rare to remove a hugetlb page from the page cache while it's > still mapped. So we don't need to worry about scalability at that point. > > Any volunteers to prototype this? Andres is a delight to work with, > but I just don't have time to take on this project right now. Hmmm ... do we really want to make refcounting more complicated, and more importantly, hugetlb-refcounting more special ?! :) If the workload doing a lot of single-page try_grab_folio_fast(), could it do so on a larger area (multiple pages at once -> single refcount update)? Maybe there is a link to the report you could share, thanks. -- Cheers, David / dhildenb