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Re: XFS thread inflation in 2.6.23rc

To: David Chinner <dgc@xxxxxxx>
Subject: Re: XFS thread inflation in 2.6.23rc
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Fri, 10 Aug 2007 14:34:10 -0500
Cc: Andi Kleen <ak@xxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20070809110322.GA12413810@xxxxxxx>
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David Chinner wrote:

> Andi, you are complaining to the wrong person about thread counts. I
> live in the world of excessive parallelism and multithreaded I/O. I
> regularly see 4-8p boxes running hundreds to thousands of I/O
> threads on a single filesystem. These are the workloads XFS is
> designed for and we optimise for. A single extra thread is noise....


The other thing this is doing, though, is having an impact on power
usage, I think.  Fedora is trying to identify things which wake the CPU
in an effort to reduce power usage if possible.

See for example:
http://article.gmane.org/gmane.linux.redhat.fedora.testers/50480

quoting:

"One of the biggest power problems is apps that wake up the CPU
unnecessarily. With the tickless kernel for x86 (and soon x86_64),
every time this happens it's a chance for power savings lost.

HOW YOU CAN HELP
----------------

We'd like to get as many reports of misbehaving apps as possible.

1) Install the 'powertop' package

2) Run it in a terminal window on a reasonably idle system with your
   normal combination of apps. (If you run it when you're actively
   compiling a kernel, watching a movie, or doing other CPU-intensive
   things, the results aren't as useful.)

3) Note the results..."

and here are the results w/ xfs mounted:

< Detailed C-state information is only available on Mobile CPUs (laptops) >
P-states (frequencies)
  2.21 Ghz     0.0%
  2.00 Ghz     0.0%
  1.80 Ghz   100.0%
Wakeups-from-idle per second : 31.7
no ACPI power usage estimate available
Top causes for wakeups:
  37.0% ( 20.0)             mount : xfs_mru_cache_create

...

looks like having this running (note: i'm not using filestreams on this
mounted filesystem) may be impacting the power usage of the CPU?  Or
maybe SGI doesn' care about that, if the goal is 100% utilization of
these machines on altixes etc... but still - something to consider I think.

Thanks,
-Eric


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