On Fri, Oct 26, 2012 at 03:19:57PM +0800, Ni zhan Chen wrote: > 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? It's this line in shrink_readahead_size_eio(): ra->ra_pages /= 4; That ra_pages will keep shrinking by 4 on each error. The only way to restore it is to reopen the file, or POSIX_FADV_SEQUENTIAL. Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html