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

[Herbert Poetzl <herbert@13thfloor.at>]

On Tue, Jan 31, 2012 at 09:48:42AM +0100, Eric Dumazet wrote:
> Le mardi 31 janvier 2012 a 15:59 +0800, Shaohua Li a ecrit :
>> 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@intel.com>
>> Tested-by: Herbert Poetzl <herbert@13thfloor.at>
>> Tested-by: Eric Dumazet <eric.dumazet@gmail.com>

> Hmm, this is not exactly the patch I tested from Wu Fengguang 

> I'll test this one before adding my "Tested-by: ..."

second that

>> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 97f49ed..b662757 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -1400,15 +1400,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
>>  	unsigned long seg = 0;
>>  	size_t count;
>>  	loff_t *ppos = &iocb->ki_pos;
>> -	struct blk_plug plug;

>>  	count = 0;
>>  	retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
>>  	if (retval)
>>  		return retval;

>> -	blk_start_plug(&plug);
>> -
>>  	/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
>>  	if (filp->f_flags & O_DIRECT) {
>>  		loff_t size;
>> @@ -1424,8 +1421,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
>>  			retval = filemap_write_and_wait_range(mapping, pos,
>>  					pos + iov_length(iov, nr_segs) - 1);
>>  			if (!retval) {
>> +				struct blk_plug plug;
>> +
>> +				blk_start_plug(&plug);

> This part was not on the tested patch yesterday.

yep, will test this one tonight ...

>>  				retval = mapping->a_ops->direct_IO(READ, iocb,
>>  							iov, pos, nr_segs);
>> +				blk_finish_plug(&plug);
>>  			}
>>  			if (retval > 0) {
>>  				*ppos = pos + retval;
>> @@ -1481,7 +1482,6 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
>>  			break;
>>  	}
>>  out:
>> -	blk_finish_plug(&plug);
>>  	return retval;
>>  }
>>  EXPORT_SYMBOL(generic_file_aio_read);

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