Re: [RFC 0/6] x86: prefetch_page() vDSO call

Nadav Amit
2021-02-25T18:02:43Z

> On Feb 25, 2021, at 9:32 AM, Matthew Wilcox <willy@infradead.org> 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@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.

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.