Re: [PATCH] readahead:add blk_run_backing_dev

Hisashi Hifumi
2009-05-27T06:25:01Z

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@intel.com>
>> >
>> > 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.