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

[Coly Li <i@coly.li>]

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@taobao.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.

BTW, if there is any performance number, it should be interesting, too.

Thanks.

-- 
Coly Li