Re: [PATCH 3/3] readahead: scale max readahead size depending on memory size

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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