Messages in this thread | | | Date | Sun, 22 Jul 2007 10:50:41 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH 3/3] readahead: scale max readahead size depending on memory size |
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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. I agree with the low point of 128k. 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.
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!
-- Jens Axboe
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