linux-kernel - Re: [patch] aio: add per task aio wait event condition

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Message-Id: <96568BA6-9ECD-4E1D-B5B3-3AA41463A8EE@oracle.com>
Date:	Tue, 2 Jan 2007 18:06:25 -0800
From:	Zach Brown <zach.brown@...cle.com>
To:	"Chen, Kenneth W" <kenneth.w.chen@...el.com>
Cc:	"'Andrew Morton'" <akpm@...l.org>, <linux-aio@...ck.org>,
	<linux-kernel@...r.kernel.org>,
	"'Benjamin LaHaise'" <bcrl@...ck.org>, <suparna@...ibm.com>
Subject: Re: [patch] aio: add per task aio wait event condition


On Jan 2, 2007, at 5:50 PM, Chen, Kenneth W wrote:

> Zach Brown wrote on Tuesday, January 02, 2007 5:24 PM
>>> That is not possible because when multiple tasks waiting for
>>> events, they
>>> enter the wait queue in FIFO order, prepare_to_wait_exclusive() does
>>> __add_wait_queue_tail().  So first io_getevents() with min_nr of 2
>>> will be woken up when 2 ops completes.
>>
>> So switch the order of the two sleepers in the example?
>
> Not sure why that would be a problem though:  whoever sleep first will
> be woken up first.

Why would the min_nr = 3 sleeper be woken up in that case?  Only 2  
ios were issued.

Maybe the app was relying on the min_nr = 2 completion to issue 3  
more ios for the min_nr = 3 sleeper, who knows.

> Before I challenge that semantics, I want to mention that in current
> implementation, dribbling AIO events will be distributed in round  
> robin
> fashion to all pending tasks waiting in io_getevents.

Yeah, don't misunderstand me -- we agree that the current situation  
is bad.

>   In the example you
> gave earlier, task with min_nr of 2 will be woken up after 4 completed
> events.

I only gave 2 ios/events in that example.

Does that clear up the confusion?

- z
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