At 13:36 09/05/27, Wu Fengguang wrote: >On Wed, May 27, 2009 at 12:06:12PM +0800, KOSAKI Motohiro wrote: >> > > Ah. So it's likely to be some strange interaction with the RAID setup. >> > >> > The normal case is, if page N become uptodate at time T(N), then >> > T(N) <= T(N+1) holds. But for RAID, the data arrival time depends on >> > runtime status of individual disks, which breaks that formula. So >> > in do_generic_file_read(), just after submitting the async readahead IO >> > request, the current page may well be uptodate, so the page won't be locked, >> > and the block device won't be implicitly unplugged: >> >> Hifumi-san, Can you get blktrace data and confirm Wu's assumption? > >To make the reasoning more obvious: > >Assume we just submitted readahead IO request for pages N ~ N+M, then > T(N) <= T(N+1) > T(N) <= T(N+2) > T(N) <= T(N+3) > ... > T(N) <= T(N+M) (M = readahead size) >So if the reader is going to block on any page in the above chunk, >it is going to first block on page N. > >With RAID (and NFS to some degree), there is no strict ordering, >so the reader is more likely to block on some random pages. > >In the first case, the effective async_size = M, in the second case, >the effective async_size <= M. The more async_size, the more degree of >readahead pipeline, hence the more low level IO latencies are hidden >to the application. I got your explanation especially about RAID specific matters. > >Thanks, >Fengguang > >> >> > >> > if (PageReadahead(page)) >> > page_cache_async_readahead() >> > if (!PageUptodate(page)) >> > goto page_not_up_to_date; >> > //... >> > page_not_up_to_date: >> > lock_page_killable(page); >> > >> > >> > Therefore explicit unplugging can help, so >> > >> > Acked-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> >> > >> > The only question is, shall we avoid the double unplug by doing this? >> > >> > --- >> > mm/readahead.c | 10 ++++++++++ >> > 1 file changed, 10 insertions(+) >> > >> > --- linux.orig/mm/readahead.c >> > +++ linux/mm/readahead.c >> > @@ -490,5 +490,15 @@ page_cache_async_readahead(struct addres >> > >> > /* do read-ahead */ >> > ondemand_readahead(mapping, ra, filp, true, offset, req_size); >> > + >> > + /* >> > + * Normally the current page is !uptodate and lock_page() will be >> > + * immediately called to implicitly unplug the device. However this >> > + * is not always true for RAID conifgurations, where data arrives >> > + * not strictly in their submission order. In this case we need to >> > + * explicitly kick off the IO. >> > + */ >> > + if (PageUptodate(page)) >> > + blk_run_backing_dev(mapping->backing_dev_info, NULL); >> > } >> > EXPORT_SYMBOL_GPL(page_cache_async_readahead); I am for this to avoid double unplug. Thanks. -- 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
