Messages in this thread | | | Date | Thu, 25 Feb 2021 17:32:53 +0000 | From | Matthew Wilcox <> | Subject | Re: [RFC 0/6] x86: prefetch_page() vDSO call |
| |
On Thu, Feb 25, 2021 at 04:56:50PM +0000, Nadav Amit wrote: > > > On Feb 25, 2021, at 4:16 AM, Matthew Wilcox <willy@infradead.org> 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
| |