On Sun, 2007-07-22 at 10:50 +0200, Jens Axboe wrote:
> On Sun, Jul 22 2007, Peter Zijlstra wrote:
> > 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?
>
> I just wish you had a rationale behind them, I don't think it's that
> great of a series.
Well, I was quite ignorant of the issues you just pointed out. Thanks
those do indeed provide basis for a more solid set.
> I agree with the low point of 128k.
Perhaps that should be enforced then, because currently a system with
<64M will get less.
> Then it'd be sane
> to try and determine what the upper limit of ra window size goodness is,
> which is probably impossible since it depends on the hardware a lot. But
> lets just say the upper value is 2mb, then I think it's pretty silly
> _not_ to use 2mb on a 1g machine for instance. So more aggressive
> scaling.
Right, I was being a little conservative here.
> Then there's the relationship between nr of requests and ra size. When
> you leave everything up to a simple sqrt of total_ram type thing, then
> you are sure to hit stupid values that cause a queue size of a number of
> full requests, plus a small one at the end. Clearly not optimal!
And this is where Wu's point of power of two series comes into play,
right?
So something like:
roundup_pow_of_two(int_sqrt((totalram_pages << (PAGE_SHIFT-10))))
memory in MB RA window in KB
64 128
128 256
256 256
512 512
1024 512
2048 1024
4096 1024
8192 2048
16384 2048
32768 4096
65536 4096
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