Re: [PATCH] readahead:add blk_run_backing_dev

KOSAKI Motohiro
2009-05-28T02:23:23Z

> Hi Andrew.
> Please merge following patch.
> Thanks.
>
> ---
>
> I added blk_run_backing_dev on page_cache_async_readahead
> so readahead I/O is unpluged to improve throughput on
> especially RAID environment.
>
> Following is the test result with dd.
>
> #dd if=testdir/testfile of=/dev/null bs=16384
>
> -2.6.30-rc6
> 1048576+0 records in
> 1048576+0 records out
> 17179869184 bytes (17 GB) copied, 224.182 seconds, 76.6 MB/s
>
> -2.6.30-rc6-patched
> 1048576+0 records in
> 1048576+0 records out
> 17179869184 bytes (17 GB) copied, 206.465 seconds, 83.2 MB/s
>
> My testing environment is as follows:
> Hardware: HP DL580
> CPU:Xeon 3.2GHz *4 HT enabled
> Memory:8GB
> Storage: Dothill SANNet2 FC (7Disks RAID-0 Array)
>
> The normal case is, if page N become uptodate at time T(N), then
> T(N) <= T(N+1) holds. With RAID (and NFS to some degree), there
> is no strict ordering, 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:

Please attach blktrace analysis ;)

>
> 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.
>
> Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>
> Acked-by: Wu Fengguang <fengguang.wu@intel.com>
>
>
> 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);