linux-kernel - Re: [PATCH][RFC] A readahead complete notify approach to implement buffer aio

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Date:	Thu, 03 Nov 2011 14:01:55 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	i@...y.li
Cc:	Zhu Yanhai <zhu.yanhai@...il.com>, linux-kernel@...r.kernel.org,
	bcrl@...ck.org, viro@...iv.linux.org.uk, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, jaxboe@...ionio.com,
	Zhu Yanhai <gaoyang.zyh@...bao.com>
Subject: Re: [PATCH][RFC] A readahead complete notify approach to implement buffer aio

Coly Li <i@...y.li> writes:

> On 2011年11月01日 17:00, Zhu Yanhai Wrote:
>> The current libaio/aio has to be Direct-IO, otherwise it falls back into sync IO.
>> However, the aio core has already been asychronous naturally. This patch adds a complete
>> notify mechanism to implement buffer aio, the main idea is to readahead()-like in
>> io_submit(), counts the non-uptodated pages assocaiated with each iocb, then put each ref
>> in the bio complete path just before unlock_page(), and hook them on to the aio ring buffer
>> finally when the ref drops to zero. In io_getevents(), we call vfs_read() as a safe net
>> since there is still little possibility that the pages had brought in were reclaimed
>> between io_submit() and io_getevents().
>> 
>> I have tested this patch for a while, for the small size random io request, its
>> performance is more or less the same with the traditional aio, for the big io request,
>> the overhead of one extra memory copy arises.
>> 
>> I think so far it has at least below obvious drawbacks,
>> 
>> * mpage_readpage() is a really narrow interface, I have no way to pass down
>> the new control struct baiocb, so I just put it into struct task_struct and
>> refer it by current() as a workaround.
>> 
>> * the do_baio_read() routine is heavily similar with do_generic_file_read(), but
>> the latter is really hard to modify. I think we may stuff these code down into the
>> readahead path to reduce code reduplication.
>> 
>> Hopefully the explanations are clear enough and don't muddy the water any worse.
>> I figure the code does need some better comments, and any suggestion are welcome.
>> 
>> Signed-off-by: Zhu Yanhai <gaoyang.zyh@...bao.com>
>> 
>> ---
>>  fs/aio.c                    |  319 ++++++++++++++++++++++++++++++++++++++++++-
>>  fs/buffer.c                 |   26 ++++-
>>  fs/mpage.c                  |   28 ++++-
>>  include/linux/aio.h         |    9 ++
>>  include/linux/aio_abi.h     |    1 +
>>  include/linux/blk_types.h   |    2 +
>>  include/linux/buffer_head.h |    3 +
>>  include/linux/page-flags.h  |    2 +
>>  include/linux/sched.h       |    1 +
>>  9 files changed, 386 insertions(+), 5 deletions(-)
>> 
>
> Hmm, I don't see the usage from user space. Is it possible to post a demo code in user space, so people are able to
> understand how to use/test your patch.

He added a new IOCB_CMD_blah for buffered aio reads.  That is,
unfortunately, a really poor way to go about doing things.  Please take
a look at the work Jens did on implementing buffered aio.  It can be
found in his linux-block git tree.

Cheers,
Jeff
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