linux-kernel - Re: trying to understand READ_META, READ_SYNC, WRITE

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Message-ID: <20100626100526.GA27159@lst.de>
Date:	Sat, 26 Jun 2010 12:05:26 +0200
From:	Christoph Hellwig <hch@....de>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	czoccolo@...il.com
Subject: Re: trying to understand READ_META, READ_SYNC, WRITE_SYNC & co

On Fri, Jun 25, 2010 at 11:35:10PM -0400, Vivek Goyal wrote:
> This logic was introduced by corrado to ensure WRITE_SYNC does not
> lose fair share. Now we are back to the same question, what is the workload
> which does that.
> 
> 8e55063 cfq-iosched: fix corner cases in idling logic
> 
> Before this patch, we will simply not do any idling on WRITE_SYNC. That
> means no idling on O_SYNC/fsync paths but still idle on direct IO
> WRITE_SYNC. Which is a bit of discrepancy.

Corrado, can you explain what workloads you doing that commit for?
The commit message doesn't really given any useful information.


> - Stop idling on all the WRITE_SYNC IO. There is no reasonable way to 
>   tell whether there will be more IO or not from applicatoin. This will
>   impact direct writes, O_SYNC writes and fsync().
> 
>   If direct IO application is submitting writes with a delay in between
>   it can be starved out in presnce of competing workloads.

So what application does this?

> - Do idling by default on WRITE_SYNC path. Implement Jeff's queue yielding
>   patches. That way O_SYNC/fsyn path will call blk_yield() and stop idling.
>   Direct IO write path will stil continue to idle (I think if file has already
>   been laid out?). 

I don't think this makes much sense.  And O_SYNC write / fsync are
defined to not return before the I/O makes it to disk, and for any
sane filesystem end with a WRITE_BARRIER request because of that.
So we end these with an explicit unplug anyway, no need for idling
logic.

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