Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

[Ni zhan Chen]

On 10/26/2012 03:09 PM, Fengguang Wu wrote:
> On Fri, Oct 26, 2012 at 03:03:12PM +0800, Ni zhan Chen wrote:
> > On 10/26/2012 02:58 PM, Fengguang Wu wrote:
> > > >   static void shrink_readahead_size_eio(struct file *filp,
> > > >                                          struct file_ra_state *ra)
> > > >   {
> > > > -       ra->ra_pages /= 4;
> > > > +       spin_lock(&filp->f_lock);
> > > > +       filp->f_mode |= FMODE_RANDOM;
> > > > +       spin_unlock(&filp->f_lock);
> > > >
> > > > As the example in comment above this function, the read maybe still
> > > > sequential, and it will waste IO bandwith if modify to FMODE_RANDOM
> > > > directly.
> > > Yes immediately disabling readahead may hurt IO performance, the
> > > original '/ 4' may perform better when there are only 1-3 IO errors
> > > encountered.
> > Hi Fengguang,
> >
> > Why the number should be 1-3?
> The original behavior is '/= 4' on each error.
>
> After 1 errors, readahead size will be shrinked by 1/4
> After 2 errors, readahead size will be shrinked by 1/16
> After 3 errors, readahead size will be shrinked by 1/64
> After 4 errors, readahead size will be effectively 0 (disabled)

But from function shrink_readahead_size_eio and its caller filemap_fault 
I can't find the behavior you mentioned. How you figure out it?

Regards,
Chen

> Thanks,
> Fengguang

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