linux-kernel - Re: linux io_submit syscall duration

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Message-ID: <20061115094727.GJ23770@kernel.dk>
Date:	Wed, 15 Nov 2006 10:47:27 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"Raz Ben-Jehuda(caro)" <raziebe@...il.com>
Cc:	inux-aio@...ck.org, linux-kernel@...r.kernel.org, bcrl@...ck.org,
	zach.brown@...cle.com
Subject: Re: linux io_submit syscall duration


(don't top post and don't drop the cc list!)

On Wed, Nov 15 2006, Raz Ben-Jehuda(caro) wrote:
> first thank you from your quick reply.
> 
> I looked and what I found is :
> "io_submit() taking a long time to return ?" from 2006-07-14.

Subject is "async I/O seems to be blocking on 2.6.15" and the posting is
from 2006-11-3. I said about a week ago, not 4 months.

> 1.  It is NOT the first IO. already took care for that in my test.

That was a different problem.

> 2.  The test uses block device and does not walk through a file system.

So would not apply.

> 3.  Taking a look at the code, it seems that the main problem is the fact
>     that in io_submit_one all the pending requests are being rerun.
> 
>    io_submit_one()
>   {
>    ...
>        if (!list_empty(&ctx->run_list)) {
>                /* drain the run list */
>              while (__aio_run_iocbs(ctx))
>                      ;
>      }
>   ...
>   }
> 
>  This code was put in remarks.
> the second is in:
>  aio_run_iocb()
>   ...
>    current->io_wait = &iocb->ki_wait;
>    if ( iocb->ki_retried >  1)
>        ret = retry(iocb);
>    else{
>        ret = -EIOCBRETRY;
>        kiocbSetKicked(iocb);
>    }
>     current->io_wait = NULL;
>  ....
> 
>  With these fixes io_submit delay reduced to zero ms.
> 
> I would appreciate your review on these fixes.

Post a patch so it can be reviewed.

-- 
Jens Axboe

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