Re: [PATCH 00/23] per device dirty throttling -v8

[Bill Davidsen <davidsen@tmr.com>]

david@lang.hm wrote:
> On Sun, 5 Aug 2007, Diego Calleja wrote:
>
> > El Sun, 5 Aug 2007 09:13:20 +0200, Ingo Molnar <mingo@elte.hu> escribió:
> >
> > > Measurements show that noatime helps 20-30% on regular desktop
> > > workloads, easily 50% for kernel builds and much more than that (in
> > > excess of 100%) for file-read-intense workloads. We cannot just walk
> >
> >
> > And as everybody knows in servers is a popular practice to disable it.
> > According to an interview to the kernel.org admins....
> >
> > "Beyond that, Peter noted, "very little fancy is going on, and that is 
> > good
> > because fancy is hard to maintain." He explained that the only fancy 
> > thing
> > being done is that all filesystems are mounted noatime meaning that the
> > system doesn't have to make writes to the filesystem for files which are
> > simply being read, "that cut the load average in half."
> >
> > I bet that some people would consider such performance hit a bug...
>
> actually, it's popular practice to disable it by people who know how big 
> a hit it is and know how few programs use it.
>
> i've been a linux sysadmin for 10 years, and have known about noatime 
> for at least 7 years, but I always thought of it in the catagory of 'use 
> it only on your performance critical machines where you are trying to 
> extract every ounce of performance, and keep an eye out for things 
> misbehaving'
>
> I never imagined that itwas the 20%+ hit that is being described, and 
> with so little impact, or I would have switched to it across the board 
> years ago.
To get that magnitude you need slow disk with very fast CPU. It helps 
most of systems where the disk hardware is marginal or worse for the i/o 
load. Don't take that as typical.

> I'll bet there are a lot of admins out there in the same boat.
>
> adding an option in the kernel to change the default sounds like a very 
> good first step, even if the default isn't changed today.

--
Bill Davidsen <davidsen@tmr.com>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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