Re: [RFC 0/6] x86: prefetch_page() vDSO call
From: Matthew Wilcox
Date: Thu Feb 25 2021 - 12:33:52 EST
On Thu, Feb 25, 2021 at 04:56:50PM +0000, Nadav Amit wrote:
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> On Feb 25, 2021, at 4:16 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
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> On Wed, Feb 24, 2021 at 11:29:04PM -0800, Nadav Amit wrote:
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>> Just as applications can use prefetch instructions to overlap
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>> computations and memory accesses, applications may want to overlap the
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>> page-faults and compute or overlap the I/O accesses that are required
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>> for page-faults of different pages.
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> Isn't this madvise(MADV_WILLNEED)?
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Good point that I should have mentioned. In a way prefetch_page() a
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combination of mincore() and MADV_WILLNEED.
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There are 4 main differences from MADV_WILLNEED:
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1. Much lower invocation cost if the readahead is not needed: this allows
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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.
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2. Return value: return value tells you whether the page is accessible.
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This makes it usable for coroutines, for instance. In this regard the
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call is more similar to mincore() than MADV_WILLNEED.
I don't quite understand the programming model you're describing here.
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3. The PTEs are mapped if the pages are already present in the
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swap/page-cache, preventing an additional page-fault just to map them.
We could enhance madvise() to do this, no?
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4. Avoiding heavy-weight reclamation on low memory (this may need to
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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