Re: [PATCH] fix readahead pipeline break caused by block plug

[Christoph Hellwig]

On Tue, Jan 31, 2012 at 03:59:40PM +0800, Shaohua Li wrote:
> Herbert Poetzl reported a performance regression since 2.6.39. The test
> is a simple dd read, but with big block size. The reason is:
> 
> T1: ra (A, A+128k), (A+128k, A+256k)
> T2: lock_page for page A, submit the 256k
> T3: hit page A+128K, ra (A+256k, A+384). the range isn't submitted
> because of plug and there isn't any lock_page till we hit page A+256k
> because all pages from A to A+256k is in memory
> T4: hit page A+256k, ra (A+384, A+ 512). Because of plug, the range isn't
> submitted again.
> T5: lock_page A+256k, so (A+256k, A+512k) will be submitted. The task is
> waitting for (A+256k, A+512k) finish.
> 
> There is no request to disk in T3 and T4, so readahead pipeline breaks.
> 
> We really don't need block plug for generic_file_aio_read() for buffered
> I/O. The readahead already has plug and has fine grained control when I/O
> should be submitted. Deleting plug for buffered I/O fixes the regression.
> 
> One side effect is plug makes the request size 256k, the size is 128k
> without it. This is because default ra size is 128k and not a reason we
> need plug here.
> 
> Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx>
> Tested-by: Herbert Poetzl <herbert@xxxxxxxxxxxx>
> Tested-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>

Please also CC -stable on this.

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