Re: [Lsf-pc] [LSF/MM TOPIC] async buffered diskio read for userspace apps

[Milosz Tanski <milosz@adfin.com>]

On Thu, Jan 15, 2015 at 5:31 PM, Jan Kara <jack@suse.cz> wrote:
> On Thu 15-01-15 12:43:23, Milosz Tanski wrote:
>> I would like to talk about enhancing the user interfaces for doing
>> async buffered disk IO for userspace applications. There's a whole
>> class of distributed web applications (most new applications today)
>> that would benefit from such an API. Most of them today rely on
>> cobbling one together in user space using a threadpool.
>>
>> The current in kernel AIO interfaces that only support DIRECTIO, they
>> were generally designed by and for big database vendors. The consensus
>> is that the current AIO interfaces usually lead to decreased
>> performance for those app.
>>
>> I've been developing a new read syscall that allows non-blocking
>> diskio read (provided that data is in the page cache). It's analogous
>> to what exists today in the network world with recvmsg with MSG_NOWAIT
>> flag. The work has been previously described by LWN here:
>> https://lwn.net/Articles/612483/
>>
>> Previous attempts (over the last 12+ years) at non-blocking buffered
>> diskio has stalled due to their complexity. I would like to talk about
>> the problem, my solution, and get feedback on the course of action.
>>
>> Over the years I've been building the low level guys of various "web
>> applications". That usually involves async network based applications
>> (epoll based servers) and the biggest pain point for the last 8+ years
>> has been async disk IO.
>   Maybe this topic will be sorted out before LSF/MM. I know Andrew had some
> objections about doc and was suggesting a solution using fincore() (which
> Christoph refuted as being racy). Also there was a pending question
> regarding whether the async read in this form will be used by applications.
> But if it doesn't get sorted out a short session on the pending issues
> would be probably useful.
>
>                                                                 Honza
> --
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR

I've spent the better part of yesterday wrapping up the first cut of
samba support to FIO so we can test a modified samba file server with
these changes in a few scenarios. Right now it's only sync but I hope
to have async in the future. I hope that by the time the summit rolls
around I'll have data to share from samba and maybe some other common
apps (node.js / twisted).

-- 
Milosz Tanski
CTO
16 East 34th Street, 15th floor
New York, NY 10016

p: 646-253-9055
e: milosz@adfin.com

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