Re: [RFC] start_aggressive_readahead

[Andrew Morton <akpm@zip.com.au>]

Vladimir Dergachev wrote:
> 
> > > > - It shrinks the window size in response to "misses"  - if
> > > >   userspace requests a page which is *not* inside the previously-requested
> > > >   window, the future window size is shrunk by 25%
> > >
> > > This one seems wierd.  If I reference a page that could have been in a
> > > larger read-ahead window, shouldn't I make the window *larger* so that
> > > next time, it *will* be in the window?
> >
> > That's true.  If the application is walking across a file
> > touching every fifth page, readahead will stabilise at
> > its minimum window size, which is less than five pages and
> > we lose bigtime.   I'm not sure how to fix that while retaining
> > some sanity in the code.
> 
> I am curious: which applications do you know of that actually do this ?

None.  Just a test program which I used for testing readahead!

> What about growing the window even if there is a miss as long as misses
> are sequential and not further than a fixed amount from the window ?

That would work.  If the window size is less than max, and the
miss occurred inside the max, increase the window to a size which
would have caught that page.  Or to the max.

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