linux-kernel - Re: [PATCH 2/2] UBIFS: start using hrtimers

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Date:	Fri, 29 May 2009 16:58:10 +0300
From:	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Steven Rostedt <srostedt@...hat.com>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH 2/2] UBIFS: start using hrtimers

Peter Zijlstra wrote:
> On Fri, 2009-05-29 at 18:09 +0300, Artem Bityutskiy wrote:
>> From: Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
>> Subject: [PATCH 2/2] UBIFS: start using hrtimers
>>
>> UBIFS uses timers for write-buffer write-back. It is not
>> crucial for us to write-back exactly on time. We are fine
>> to write-back a little earlier or later. And this means
>> we may optimize UBIFS timer so that it could be groped
>> with a close timer event, so that the CPU would not be
>> waken up just to do the write back. This is optimization
>> to lessen power consumption, which is important in
>> embedded devices UBIFS is used for.
>>
>> hrtimers have a nice feature: they are effectively range
>> timers, and we may defind the soft and hard limits for
>> it. Standard timers do not have these feature. They may
>> only be made deferrable, but this means there is effectively
>> no hard limit. So, we will better use hrtimers.
>>
>> Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
>> ---
> 
> Seems sane enough from a hrtimer POV, but isn't this already
> functionality that the VFS/pdflush provide?

Yeah, VFS/pdflush takes care of the page-cache and inode cache,
and dirty superblocks. But additionally to this UBIFS has its
own small buffer of (usually) 2KiB size, we call it write-buffer.
This is a very important optimization for NAND flash. And we
have a separate timer to synchronize this small write-buffer.

I was also thinking to hook to VFS, which would mean creating
a fake inode representing our write-bufffer. But this would be
very hacky.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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