On 17/07/2024 11:31, David Hildenbrand wrote: > On 17.07.24 09:12, Ryan Roberts wrote: >> Hi All, >> >> This series is an RFC that adds sysfs and kernel cmdline controls to configure >> the set of allowed large folio sizes that can be used when allocating >> file-memory for the page cache. As part of the control mechanism, it provides >> for a special-case "preferred folio size for executable mappings" marker. >> >> I'm trying to solve 2 separate problems with this series: >> >> 1. Reduce pressure in iTLB and improve performance on arm64: This is a modified >> approach for the change at [1]. Instead of hardcoding the preferred executable >> folio size into the arch, user space can now select it. This decouples the arch >> code and also makes the mechanism more generic; it can be bypassed (the default) >> or any folio size can be set. For my use case, 64K is preferred, but I've also >> heard from Willy of a use case where putting all text into 2M PMD-sized folios >> is preferred. This approach avoids the need for synchonous MADV_COLLAPSE (and >> therefore faulting in all text ahead of time) to achieve that. >> >> 2. Reduce memory fragmentation in systems under high memory pressure (e.g. >> Android): The theory goes that if all folios are 64K, then failure to allocate a >> 64K folio should become unlikely. But if the page cache is allocating lots of >> different orders, with most allocations having an order below 64K (as is the >> case today) then ability to allocate 64K folios diminishes. By providing control >> over the allowed set of folio sizes, we can tune to avoid crucial 64K folio >> allocation failure. Additionally I've heard (second hand) of the need to disable >> large folios in the page cache entirely due to latency concerns in some >> settings. These controls allow all of this without kernel changes. >> >> The value of (1) is clear and the performance improvements are documented in >> patch 2. I don't yet have any data demonstrating the theory for (2) since I >> can't reproduce the setup that Barry had at [2]. But my view is that by adding >> these controls we will enable the community to explore further, in the same way >> that the anon mTHP controls helped harden the understanding for anonymous >> memory. >> >> --- > > How would this interact with other requirements we get from the filesystem (for > example, because of the device) [1]. > > Assuming a device has a filesystem has a min order of X, but we disable anything >>= X, how would we combine that configuration/information? Currently order-0 is implicitly the "always-on" fallback order. My thinking was that with [1], the specified min order just becomes that "always-on" fallback order. Today: orders = file_orders_always() | BIT(0); Tomorrow: orders = (file_orders_always() & ~(BIT(min_order) - 1)) | BIT(min_order); That does mean that in this case, a user-disabled order could still be used. So the controls are really hints rather than definitive commands. > > > [1] > https://lore.kernel.org/all/20240715094457.452836-2-kernel@xxxxxxxxxxxxxxxx/T/#u >
