Re: [PATCH] readahead:add blk_run_backing_dev

[Hisashi Hifumi]

>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.
>
>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?
>> > 


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:

               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@xxxxxxxxxxxxx>
Acked-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> 


 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); 

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