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SubjectRe: [PATCH 3/3] readahead: scale max readahead size depending on memory size
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On Sun, 2007-07-22 at 10:24 +0200, Jens Axboe wrote:
> On Sat, Jul 21 2007, Peter Zijlstra wrote:

> > +static __init int readahead_init(void)
> > +{
> > + /*
> > + * Scale the max readahead window with system memory
> > + *
> > + * 64M: 128K
> > + * 128M: 180K
> > + * 256M: 256K
> > + * 512M: 360K
> > + * 1G: 512K
> > + * 2G: 724K
> > + * 4G: 1024K
> > + * 8G: 1448K
> > + * 16G: 2048K
> > + */
> > + ra_pages = int_sqrt(totalram_pages/16);
> > + if (ra_pages > (2 << (20 - PAGE_SHIFT)))
> > + ra_pages = 2 << (20 - PAGE_SHIFT);
> > +
> > + return 0;
> > +}
>
> How did you come up with these numbers?

Well, most other places in the kernel where we scale by memory size we
use the a sqrt curve, and the specific scale was the result of some
fiddling, these numbers looked sane to me, nothing special.

Would you suggest a different set, and if so, do you have any rationale
for them?

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