Messages in this thread | | | Subject | Re: [PATCH 3/3] readahead: scale max readahead size depending on memory size | From | Peter Zijlstra <> | Date | Sun, 22 Jul 2007 11:17:32 +0200 |
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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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