On Fri, Oct 26, 2012 at 11:38:11AM +0800, YingHang Zhu wrote: > On Fri, Oct 26, 2012 at 8:25 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Thu, Oct 25, 2012 at 10:58:26AM +0800, Fengguang Wu wrote: > >> Hi Chen, > >> > >> > But how can bdi related ra_pages reflect different files' readahead > >> > window? Maybe these different files are sequential read, random read > >> > and so on. > >> > >> It's simple: sequential reads will get ra_pages readahead size while > >> random reads will not get readahead at all. > >> > >> Talking about the below chunk, it might hurt someone that explicitly > >> takes advantage of the behavior, however the ra_pages*2 seems more > >> like a hack than general solution to me: if the user will need > >> POSIX_FADV_SEQUENTIAL to double the max readahead window size for > >> improving IO performance, then why not just increase bdi->ra_pages and > >> benefit all reads? One may argue that it offers some differential > >> behavior to specific applications, however it may also present as a > >> counter-optimization: if the root already tuned bdi->ra_pages to the > >> optimal size, the doubled readahead size will only cost more memory > >> and perhaps IO latency. > >> > >> --- a/mm/fadvise.c > >> +++ b/mm/fadvise.c > >> @@ -87,7 +86,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice) > >> spin_unlock(&file->f_lock); > >> break; > >> case POSIX_FADV_SEQUENTIAL: > >> - file->f_ra.ra_pages = bdi->ra_pages * 2; > > > > I think we really have to reset file->f_ra.ra_pages here as it is > > not a set-and-forget value. e.g. shrink_readahead_size_eio() can > > reduce ra_pages as a result of IO errors. Hence if you have had io > > errors, telling the kernel that you are now going to do sequential > > IO should reset the readahead to the maximum ra_pages value > > supported.... > If we unify file->f_ra.ra_pages and its' bdi->ra_pages, then the error-prone > device's readahead can be directly tuned or turned off with blockdev > thus affect all files > using the device and without bring more complexity... It's not really feasible/convenient for the end users to hand tune blockdev readahead size on IO errors. Even many administrators are totally unaware of the readahead size parameter. Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>