On Fri, Oct 26, 2012 at 11:25:44AM +1100, Dave Chinner 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.... Good point! .... but wait .... this patch removes file->f_ra.ra_pages in all other places too, so there will be no file->f_ra.ra_pages to be reset here... 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>