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[2003:cb:c736:ca00:b4c3:24bd:c2f5:863c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438bd54c0ecsm144033105e9.30.2025.01.27.11.20.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Jan 2025 11:20:42 -0800 (PST) Message-ID: <4a75d25f-bcb9-42b6-aa9e-1e63e4be98e3@redhat.com> Date: Mon, 27 Jan 2025 20:20:41 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Direct I/O performance problems with 1GB pages To: Andres Freund Cc: Matthew Wilcox , linux-mm@kvack.org, 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: IlL_tJX_NH8IaIyG3unt5RPpQM6fhxfNKW0DfKEAsKw_1738005645 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: AE63540011 X-Stat-Signature: fojc9z8p7tc5p1d11gr5hrh1yf56tiuc X-HE-Tag: 1738005647-822817 X-HE-Meta: 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 p53zd6GJ 1teEiCFpafTFJm6BCObTf5VLCih8DATjx35RNlfDWg9QnBLmCDlTJkhoHAt1gSElCRTnAcWdgeX1tevEbR8DPJnqVYUj1dFt9WVwjrl4M5Zvuims8wgl4TIQ+XfUURxWSD7lngHa6hXRnt6j4Ke8YGCSV1TXl49eVupSDimCQdeo3R/GnHbKgv6S4Cedx/74Hanebop2Xs2d3AtVw+YA7M+weYDEtaxzZb0/v8ywuQE2bLJheZqmkHaH/Z5rrIj8Bzz9zjdHZPT9URnV/ttB+sxk4ee3g4aMgAdNOVu5Qc16X/2LAhreI1jWMymB9JkDcEl+oZieekZw5eIb3D6Rig/azmEDBwuTSQMr3LkBSBBm3cSmL+DGUI32hEt2LxWW1gHCk52l/z3BdyQzmtP219Zkjg+nRT2fFqzJZ 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 27.01.25 18:25, Andres Freund wrote: > Hi, > > On 2025-01-27 15:09:23 +0100, David Hildenbrand wrote: >> Hmmm ... do we really want to make refcounting more complicated, and more >> importantly, hugetlb-refcounting more special ?! :) > > I don't know the answer to that - I mainly wanted to report the issue because > it was pretty nasty to debug and initially surprising (to me). Thanks for reporting! > > >> 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)? > > In the original case I hit this I (a VM with 10 PCIe 3x NVMEs JBODed > together), the IO size averaged something like ~240kB (most 256kB, with some > smaller ones thrown in). Increasing the IO size further than that starts to > hurt latency and thus requires even deeper IO queues... Makes sense. > > Unfortunately for the VMs with those disks I don't have access to hardware > performance counters :(. > > >> Maybe there is a link to the report you could share, thanks. > > A profile of the "original" case where I hit this, without the patch that > Willy linked to: > > Note this is a profile *not* using hardware perf counters, thus likely to be > rather skewed: > https://gist.github.com/anarazel/304aa6b81d05feb3f4990b467d02dabc > (this was on Debian Sid's 6.12.6) > > Without the patch I achieved ~18GB/s with 1GB pages and ~35GB/s with 2MB > pages. Out of interest, did you ever compare it to 4k? > > After applying the patch to add an unlocked already-dirty check to > bio_set_pages_dirty() performance improves to ~20GB/s when using 1GB pages. > > A differential profile comparing 2MB and 1GB pages with the patch applied > (again, without hardware perf counters): > https://gist.github.com/anarazel/f993c238ea7d2c34f44440336d90ad8f > > > Willy then asked me for perf annotate of where in gup_fast_fallback() time is > spent. I didn't have access to the VM at that point, and tried to repro the > problem with local hardware. > > > As I don't have quite enough IO throughput available locally, I couldn't repro > the problem quite as easily. But after lowering the average IO size (which is > not unrealistic, far from every workload is just a bulk sequential scan), it > showed up when just using two PCIe 4 NVMe SSDs. > > Here are profiles of the 2MB and 1GB cases, with the bio_set_pages_dirty() > patch applied: > https://gist.github.com/anarazel/f0d0a884c55ee18851dc9f15f03f7583 > > 2MB pages get ~12.5GB/s, 1GB pages ~7GB/s, with a *lot* of variance. Thanks! > > This time it's actual hardware perf counters... > > Relevant details about the c2c report, excerpted from IRC: > > andres | willy: Looking at a bit more detail into the c2c report, it looks > like the dirtying is due to folio->_pincount and folio->_refcount in > about equal measure and folio->flags being modified in > gup_fast_fallback(). The modifications then, unsurprisingly, cause a > lot of cache misses for reads (like in bio_set_pages_dirty() and > bio_check_pages_dirty()). > > willy | andres: that makes perfect sense, thanks > willy | really, the only way to fix that is to split it up > willy | and either we can split it per-cpu or per-physical-address-range As discussed, even better is "not repeatedly pinning/unpinning" at all :) I'm curious, are multiple processes involved, or is this all within a single process? -- Cheers, David / dhildenb