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From: Andrew Morton <akpm@osdl.org>
To: dean gaudet <dean@arctic.org>
Cc: Neil Brown <neilb@suse.de>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH - RFC] allow setting vm_dirty below 1% for large memory machines
Date: Thu, 11 Jan 2007 14:48:01 -0800	[thread overview]
Message-ID: <20070111144801.ef86c169.akpm@osdl.org> (raw)
In-Reply-To: <Pine.LNX.4.64.0701111431470.4980@twinlark.arctic.org>

On Thu, 11 Jan 2007 14:35:06 -0800 (PST)
dean gaudet <dean@arctic.org> wrote:

> actually a global dirty_ratio causes interference between devices which 
> should otherwise not block each other...
> 
> if you set up a "dd if=/dev/zero of=/dev/sdb bs=1M" it shouldn't affect 
> write performance on sda -- but it does... because the dd basically 
> dirties all of the "dirty_background_ratio" pages and then any task 
> writing to sda has to block in the foreground...  (i've had this happen in 
> practice -- my hack fix is oflag=direct on the dd... but the problem still 
> exists.)

yeah.  Plus your heavy-dd-to-/dev/sda tends to block light-writers to
/dev/sda in perhaps disproportionate ways.

This is on my list of things to look at.  Hah.

> i'm not saying fixing any of this is easy, i'm just being a user griping 
> about it :)

It's rather complex, I believe.   Needs per-backing-dev dirty counts (already
in -mm) plus, I suspect, per-process dirty counts (possibly derivable from
per-task-io-accounting) plus some tricky logic to make all that work along
with global dirtiness (and later per-node dirtiness!) while meeting all the
constraints which that logic must satisfy.

  reply	other threads:[~2007-01-11 22:48 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-01-09  8:57 Neil Brown
2007-01-09 10:10 ` Andrew Morton
2007-01-10  3:04   ` Neil Brown
2007-01-10  3:29   ` Neil Brown
2007-01-10  3:41     ` Andrew Morton
2007-01-11 11:04 ` dean gaudet
2007-01-11 20:21   ` Andrew Morton
2007-01-11 22:35     ` dean gaudet
2007-01-11 22:48       ` Andrew Morton [this message]
2007-03-07 10:23         ` Leroy van Logchem

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