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Subject: Re: Random file I/O regressions in 2.6
From: Ram Pai <linuxram@us.ibm.com>
To: Andrew Morton <akpm@osdl.org>
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On Tue, 2004-05-11 at 14:17, Andrew Morton wrote:
> Ram Pai <linuxram@us.ibm.com> wrote:
 
I am yet to get my machine fully set up to run a DSS benchmark. But
thought I will update you on the following comment.
 
> 
> multiline comment layout:
> 
> 		/*
> 		 * To avoid rounding errors, ensure that 'average' tends
> 		 * towards the value of ra->serial_cnt.
> 		 */
> 
> (I said "minor").
> 
> I can't say that I immediately understand what is the issue here with
> rounding errors?

Say the i/o size is 20 pages.

Our algorithm starts by a initial average i/o size of 'ra_pages/2' which
is mostly say 16.

Now every time we take a average, the 'average' progresses as follows
(16+20)/2=18
(18+20)/2=19
(19+20)/2=19
(19+20)/2=19.....
and the rounding error makes it never touch 20

However the code can be further optimized to :

 		/* 
                 * to avoid rounding errors, ensure that 'average' 
                 * tends towards the value of ra->serial_cnt.
                 */
                if (ra->average < ra->serial_cnt) {
                        average = ra->average + 1;
                }

I will send a updated patch with all your comments incorporated as soon
as I see good benchmark numbers.(probably by tomorrow).

RP






> 
> 
> > +                if(ra->average > ra->serial_cnt) {
> 
> space between "if" and "("
> 
> > +			ra->next_size = (ra->average > max ?  
> > +				max : ra->average); 
> 
> 	min(max, ra->average) ?
> 
> 
>