linux-ext4 - Re: [GIT PULL] Ext3 latency fixes

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Date:	Mon, 6 Apr 2009 07:48:13 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Arjan van de Ven <arjan@...radead.org>,
	Theodore Tso <tytso@....edu>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [GIT PULL] Ext3 latency fixes

On Mon, 6 Apr 2009, Jens Axboe wrote:
> 
> Well, either you are submitting a single piece of IO (in which case you
> just want to unplug or directly submit as part of the submit_bio()), or
> you are submitting several IOS (in which case you just want to unplug at
> the end of the IO submission, before waiting).

That's not true.

The plugging is often across multiple threads. It didn't _use_ to be (we 
always unplugged at wait), but it is now. Nothing else explains why that 
patch by Ted makes such a big throughput thing, because the code did

      ret = submit_bh(WRITE_SYNC, bh);
      wait_on_buffer(bh);

ie it very much submits a _single_ IO, and waits on it. If plugging made a 
difference, that means that unplugging was delayed so long that somebody 
else does IO too - ie it gets delayed past a wait event.

So according to your own rules, that submit_bh() _should_ use WRITE_SYNC, 
but something bad happens if it does. I'm not quite seeing _what_, though, 
unless there are multiple processes trying to dirty the _same_ buffer, and 
they win if they all can dirty it without doing IO on it in between (and 
then the write turns into just one write).

		Linus

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