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 kA3jbJpo 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I'd like to focus on my anon_vma topic, so I'm going to have to drop this one on physical LRU scanning. If anybody else would like to lead feel free, but I think this one is rather too dependent on my providing my analysis (which unfortunately I don't have time for any longer) so I don't think this is really feasible :( I may try to pick this up in 2026, and perhaps by then it won't be a post-mortem of a failed attempt but rather a discussion of a succesful one :) let's see. Best, Lorenzo On Wed, Jan 08, 2025 at 09:46:31PM +0000, Lorenzo Stoakes wrote: > Hi all, > > Not too long ago I took some time to investigate the possibility of > scanning physical memory directly by traversing the memory map directly > rather than the LRU linked list. > > This was inspired by a post from Matthew [0] wherein he demonstrated just > how significant the difference is between traversing arrays of contiguous > data on a modern system vs. the almost worst-case scenario of traversing a > linked-list. > > I tested how this might look by implementing code which simply traverses > and filters the memory map for LRU pages, simplifying as much as possible. > > However no matter which machine (ranging from 16 GB - 192 GB) or whether > virtualised or real hardware, I found unfortunately disappointing results - > the act of having to scan such a large range of memory resulted in > performance significantly less than a typical LRU scan at low memory > utilisation and performance at best matching LRU scanning at high memory > utilisation (simulating higher memory pressure). > > There are a number of factors at play here, and perhaps the shrinkage of > struct page (allowing for denser placement in cache lines), or an improved > algorithm might lead to more promising results. > > Having discussed this with Matthew, he suggested I put forward a proposal > to discuss this area in order that we can learn from this should it appear > this approach is unworkable or perhaps determine whether there might be > something to this that we might still salvage. > > I intend to do some more research and generate some more specific numbers > (feel free to give feedback here) before LSF so we can have something more > specific to talk about. > > I always envisioned this approach being somehow integrated with MGLRU and I > wonder if some hybrid means of integrating this approach with the MGLRU one > might make sense, which could also be another area of discussion. > > Thanks! > > [0]:https://lore.kernel.org/linux-mm/ZTc7SHQ4RbPkD3eZ@casper.infradead.org/