On Thu, Feb 25, 2021 at 04:56:50PM +0000, Nadav Amit wrote: > > > On Feb 25, 2021, at 4:16 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Wed, Feb 24, 2021 at 11:29:04PM -0800, Nadav Amit wrote: > >> Just as applications can use prefetch instructions to overlap > >> computations and memory accesses, applications may want to overlap the > >> page-faults and compute or overlap the I/O accesses that are required > >> for page-faults of different pages. > > > > Isn't this madvise(MADV_WILLNEED)? > > Good point that I should have mentioned. In a way prefetch_page() a > combination of mincore() and MADV_WILLNEED. > > There are 4 main differences from MADV_WILLNEED: > > 1. Much lower invocation cost if the readahead is not needed: this allows > to prefetch pages more abundantly. That seems like something that could be fixed in libc -- if we add a page prefetch vdso call, an application calling posix_madvise() could be implemented by calling this fast path. Assuming the performance increase justifies this extra complexity. > 2. Return value: return value tells you whether the page is accessible. > This makes it usable for coroutines, for instance. In this regard the > call is more similar to mincore() than MADV_WILLNEED. I don't quite understand the programming model you're describing here. > 3. The PTEs are mapped if the pages are already present in the > swap/page-cache, preventing an additional page-fault just to map them. We could enhance madvise() to do this, no? > 4. Avoiding heavy-weight reclamation on low memory (this may need to > be selective, and can be integrated with MADV_WILLNEED). Likewise. I don't want to add a new Linux-specific call when there's already a POSIX interface that communicates the exact same thing. The return value seems like the only problem. https://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_madvise.html
