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SubjectRe: 2.4.10 does not set accessed bit for readahead pages

On Fri, 28 Sep 2001, Marcelo Tosatti wrote:
>
> I've done some experiments in the past trying to use a new GFP_ flag for
> allocation of readahead pages. This new GFP flag was the indicator to
> __alloc_pages() that it should fail very easily (so, in theory, we would
> not spend time on page reclaiming for readahead, which is simply a guess).

I don't think the above has anything to do with the accessed bit.

The fact is, failing very easily will mean that we never do much
read-ahead at all, or will mean that _other_ allocations will have to bear
more of the grunt of the work, and that the LRU lists just become
unbalanced. I doubt that the fact that your GFP experiments back up
touching the accessed bit.

However, if somebody has benchmarks comparing apples to apples (ie
comparing _just_ setting/notsetting the accessed bit), that would be very
interesting.

Not setting the accessed bit means that the read-ahead has to be useful
within "one loop" of the inactive list. I think that's the right thing: if
we don't actually touch the pages soon, the read-ahead was probably a
loss. And it's better to cut your losses early rather than late.

Linus

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