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

Bill Davidsen
2007-08-10T04:04:45Z

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> escribio:
>>
>>> 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