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Og5hAP2eTRXlOQhA6rP1rjO7LFi7mCQ7zafN3KFKZtzDhHjZiT81BXKcW4ibyuZikOeVeUTngTG1S/7KY8qhSkza/pCEUWtS/cwnYnCDrnqRcfd1C4SsIyZ26QJ7Ydw00lEYjAVU6F1twAea+lZ7xZEW/5M1aLtSKNsCgx/aee4z+eXRpdHCqiQce4uxWBeC3c2Fpe3+kFwN90N/n4GeWFO7fMdLOo54bBXjRkahE/CYQa64e9ayqpMVby/oRa7cKYXuDrItNviP6ApOLmSlr+zsy7tKF8ugv6tA1qzCLahEEYxhmaq+CZ1m/l92oYh95XG+We3lniarqlGAQxIcUX0wcxw== 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: On 3/14/23 23:01, Matthew Wilcox wrote: > On Tue, Mar 14, 2023 at 10:50:36PM +0800, Yin, Fengwei wrote: >> On 3/14/2023 5:48 PM, Matthew Wilcox wrote: >>> On Tue, Mar 14, 2023 at 10:16:09AM +0100, David Hildenbrand wrote: >>>> Just curious what the last sentence implies. Large folios are supposed to be >>>> a transparent optimization. So why should we pageout all surrounding >>>> subpages simply because a single subpage was requested to be paged out? That >>>> might harm performance of some workloads ... more than the actual split. >>>> >>>> So it's not immediately obvious to me why "avoid splitting" is the correct >>>> answer to the problem at hand. >>> >>> At least for anonymous pages, using large folios is an attempt to treat >>> all pages in a particular range the same way. If the user says to only >>> page out some of them, that's a big clue that these pages are different >>> from the other pages, and so we should split a folio where the madvise >>> call does not cover every page in the folio. >> >> Yes. This is my understanding also. :). >> >>> I'm less convinced that argument holds for page cache pages. >> >> Can you explain more about this? My understanding is that if we need >> to reclaim the large folio for page cache, it's better to reclaim the >> whole folio. > > Pagecache is a shared resource. To determine how best to handle all > the memory used to cache a file (ie the correct folio size), ideally > we would take into account how all the users of a particular file are > using it. If we just listen to the most recent advice from one user, > we risk making a decision that's bad for potentially many other users. > > Of course, we don't have any framework for deciding the correct folio size > used for pagecache yet. We have the initial guess based on readahead > and we have various paths that will split back to individual pages. > But it's something I know we'll want to do at some point. Thanks a lot for detail explanation. Regards Yin, Fengwei