> On Feb 25, 2021, at 9:32 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > 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. I agree that this call does not have to be exposed to the application. I am not sure there is a lot of extra complexity now, but obviously some evaluations are needed.
