LKML Archive on
help / color / mirror / Atom feed
From: dean gaudet <>
To: Andrew Morton <>
Cc: Neil Brown <>,
Subject: Re: [PATCH - RFC] allow setting vm_dirty below 1% for large memory machines
Date: Thu, 11 Jan 2007 14:35:06 -0800 (PST)	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, 11 Jan 2007, Andrew Morton wrote:

> On Thu, 11 Jan 2007 03:04:00 -0800 (PST)
> dean gaudet <> wrote:
> > On Tue, 9 Jan 2007, Neil Brown wrote:
> > 
> > > Imagine a machine with lots of memory - say 100Gig.
> > 
> > i've had these problems on machines as "small" as 8GiB.  the real problem 
> > is that the kernel will let millions of potential (write) IO ops stack up 
> > for a device which can handle only mere 100s of IOs per second.  (and i'm 
> > not convinced it does the IOs in a sane order when it has millions to 
> > choose from)
> > 
> > replacing the percentage based dirty_ratio / dirty_background_ratio with 
> > sane kibibyte units is a good fix... but i'm not sure it's sufficient.
> > 
> > it seems like the "flow control" mechanism (i.e. dirty_ratio) should be on 
> > a device basis...
> > 
> > try running doug ledford'd on an 8GiB box with a single disk, 
> > let it go a few minutes then ^C and type "sync".  i've had to wait 10 
> > minutes (2.6.18 with default vm settings).
> > 
> > it makes it hard to guarantee a box can shutdown quickly -- nasty for 
> > setting up UPS on-battery timeouts for example.
> > 
> Increasing the request queue size should help there
> (/sys/block/sda/queue/nr_requests).  Maybe 25% or more benefit with that
> test, at a guess.

hmm i've never had much luck with increasing nr_requests... if i get a 
chance i'll reproduce the problem and try that.

> Probably initscripts should do that rather than leaving the kernel defaults
> in place.  It's a bit tricky for the kernel to do because the decision
> depends upon the number of disks in the system, as well as the amount of
> memory.
> Or perhaps the kernel should implement a system-wide limit on the number of
> requests in flight.  While avoiding per-device starvation.  Tricky.

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 

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


  reply	other threads:[~2007-01-11 22:35 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 [this message]
2007-01-11 22:48       ` Andrew Morton
2007-03-07 10:23         ` Leroy van Logchem

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \
    --subject='Re: [PATCH - RFC] allow setting vm_dirty below 1% for large memory machines' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).