On Tue, Jul 4, 2023 at 7:20 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: > > On 03/07/2023 20:50, Yu Zhao wrote: > > On Mon, Jul 3, 2023 at 7:53 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: > >> > >> arch_wants_pte_order() can be overridden by the arch to return the > >> preferred folio order for pte-mapped memory. This is useful as some > >> architectures (e.g. arm64) can coalesce TLB entries when the physical > >> memory is suitably contiguous. > >> > >> The first user for this hint will be FLEXIBLE_THP, which aims to > >> allocate large folios for anonymous memory to reduce page faults and > >> other per-page operation costs. > >> > >> Here we add the default implementation of the function, used when the > >> architecture does not define it, which returns the order corresponding > >> to 64K. > > > > I don't really mind a non-zero default value. But people would ask why > > non-zero and why 64KB. Probably you could argue this is the large size > > all known archs support if they have TLB coalescing. For x86, AMD CPUs > > would want to override this. I'll leave it to Fengwei to decide > > whether Intel wants a different default value.> > > Also I don't like the vma parameter because it makes > > arch_wants_pte_order() a mix of hw preference and vma policy. From my > > POV, the function should be only about the former; the latter should > > be decided by arch-independent MM code. However, I can live with it if > > ARM MM people think this is really what you want. ATM, I'm skeptical > > they do. > > Here's the big picture for what I'm tryng to achieve: > > - In the common case, I'd like all programs to get a performance bump by > automatically and transparently using large anon folios - so no explicit > requirement on the process to opt-in. We all agree on this :) > - On arm64, in the above case, I'd like the preferred folio size to be 64K; > from the (admittedly limitted) testing I've done that's about where the > performance knee is and it doesn't appear to increase the memory wastage very > much. It also has the benefits that for 4K base pages this is the contpte size > (order-4) so I can take full benefit of contpte mappings transparently to the > process. And for 16K this is the HPA size (order-2). My highest priority is to get 16KB proven first because it would benefit both client and server devices. So it may be different from yours but I don't see any conflict. > - On arm64 when the process has marked the VMA for THP (or when > transparent_hugepage=always) but the VMA does not meet the requirements for a > PMD-sized mapping (or we failed to allocate, ...) then I'd like to map using > contpte. For 4K base pages this is 64K (order-4), for 16K this is 2M (order-7) > and for 64K this is 2M (order-5). The 64K base page case is very important since > the PMD size for that base page is 512MB which is almost impossible to allocate > in practice. Which case (server or client) are you focusing on here? For our client devices, I can confidently say that 64KB has to be after 16KB, if it happens at all. For servers in general, I don't know of any major memory-intensive workloads that are not THP-aware, i.e., I don't think "VMA does not meet the requirements" is a concern.
